SudoCue - User Guide



The HTML help files that I supply with SudoCue are hopelessly out of date. Sorry about that. As a true software developer, I never bother much about documentation. Not that I haven’t received many complaints. Most of the program functions are self evident. This user guide is written to make up for the shortcomings in the packaged documentation. Future releases will bring you to this website when you select the Help Index from the menu. It is easier for me to keep the documentation on the website and improve it, even after you have downloaded the program. Make this page available offline if you do not have a permanent Internet connection. Your browser should have a menu option to do that.

This documentation is up to date with version 1.5.1 of SudoCue. It was last revised on September 9, 2006.

About SudoCue

SudoCue is a freeware program that runs on Windows 98, 2000, ME, XP and whatever Microsoft names the next release. What happened to good old version numbers? You need the .Net Framework to run it. This is a runtime environment for programs developed in Visual Studio .Net. Windows XP already has the FrameWork installed, but the installer will automatically verify the installation of the FrameWork and point you to the Microsoft website where you can download it. Don’t be put off by its size. All the goodies in the FrameWork allow me to publish an installer package that is only 500Kb in size. The next release of SudoCue is therefore quicker to install. Many other software publishers develop their programs for the FrameWork, so it is inevitable that you will need it at some point in time. I recommend you install the latest version 2.0 of the FrameWork, but version 1.1 is also supported. If you have a really old Windows version, you may even need to download the Windows Installer first. This program recognizes the .msi file that you can download from my website.

Freeware means that it does not cost you any money to download, install and use the program, but I do appreciate it when you send me a donation if you like the program and use it on a regular basis. There is a PayPal donate button in the navigation bar. So far, I have received 5 donations for 15,500 downloaded copies of the program. I am really grateful for these donations.
Please read my copyright notice to see what you can and cannot do with the software from a legal point of view.

SudoCue is a program that generates sudoku puzzles in various difficulties and helps you solve them by providing a suite of tools to inspect the grid and give you useful hints. The program can solve any valid sudoku puzzle, or report the invalidity when no unique solution can be found. This makes the program a generator, helper and solver. That’s 3 programs for the price of none. Some of the functions are not found anywhere else, like the optimizer and the scrambler. The program accepts input is many formats, from file and from the clipboard. Save and copy functions are also included. It is an excellent tool to communicate sudoku problems with other players on the popular sudoku forums.

Getting started

After the installation, start SudoCue by selecting it from your programs menu. For quick access, you can copy a shortcut to your desktop or the Quick Launch folder.
You will see an empty grid. Press Control+Shift+1 to generate an easy puzzle. If you like to work with pencilmarks, press M to show them. The LEDs below the grid are now enabled. Click an LED to toggle the pencilmark for that candidate in the selected cell. Click one of the number buttons above the grid to highlight all cells that allow the selected number. Use the arrow keys to navigate through the grid. You can also click a cell to select it. Type a number between 1 and 9 to place that digit in the selected cell. Press Delete to clear a digit. Number keys from your main keyboard and from the numeric keypad can be used. Press F8 when you're stuck and need a hint. If you don’t want to use any of the advance features, stop reading here, because you’ve learned all that you need to know. Happy puzzling!

Select variant

Before you load a puzzle, you need to choose the Sudoku variant that you like to play. Only when you open a .sdk file that has been saved with version 1.5.0 or later, the variant is correctly set upon loading the puzzle. Other types of input do not have variant information present, so the program must be told which variant to expect, before you load or enter a puzzle. The current variant is shown in the status bar.

The following variants are supported by the program:

  • Standard Sudoku. 9 rows, 9 columns and 9 3x3 boxes.
  • Sudoku-X. 9 rows, 9 columns, 9 3x3 boxes and 2 diagonals.
  • Windoku (NRC). 9 rows, 9 columns, 9 3x3 boxes, 4 window panes and 5 implied groups.
  • Disjoint Groups (DG). 9 rows, 9 columns, 9 3x3 boxes and 9 disjoint groups for cells with the same relative box position.

Setup mode

To clear the grid and switch to setup mode, select File/New Setup from the menu or press Control+N. To change the existing puzzle you’re working on, press F2. In setup mode, the solver and all the helper tools are switched off. The Options dialog can always be opened from the Tools menu, even in setup mode. The status bar shows the current mode.

The most important activity in setup mode is placing the given numbers. Navigate the grid with the arrow keys or use the mouse to select a cell. A dotted rectangle shows the cursor position in the grid. Use number keys to place digits, erase them by typing a zero, blank or the Delete key. If you suffer from keyboard phobia, you can also right-click a cell and place a digit using the context menu.

The next release allows you to use the Shift and Control keys to select multiple cells.

There are two ways to leave setup mode. Select File/Save from the menu to save your work in a file. A save dialog will show, allowing you to name the file. This name is added to the title bar afterwards. When the file is successfully saved, the program will switch to play mode, allowing you to solve the puzzle. To switch to play mode without saving the puzzle, press F3. The puzzle will be validated in both cases. When the solver cannot find a unique solution, a message will appear informing you about this. You have the choice to continue play with the solver disabled. Hints will only be available for the part that the solver could manage. The better choice is to cancel and fix the problem.

Alternative input methods

You can open a file from disk. Choose File/Open or press Control+O to do this. SudoCue will recognize files with the extensions .sdk, .sdm and .ss. The last one is the Simple Sudoku format. The .sdm extension is only used by SudoCue and contains a collection of multiple sudoku puzzles. There is no limit to the maximum collection size, but collections with more than 20,000 puzzles may take a while to load. The .sdk format is also recognized by other programs, but only SudoCue will save and recognize the custom properties. Select File/Properties to view or change them.

When you encounter an interesting sudoku in text format on a forum, you can copy the puzzle from your browser, and paste it in SudoCue. Use Edit/Paste from the menu or press Control+V. The program recognizes almost anything that remotely looks like a sudoku puzzle, including candidate grids. You can test this feature by copying and pasting the following examples:

9 5 .|. . .|. 3 8
. 3 6|. . .|5 4 .
. . .|8 . 5|. . .
. 8 .|9 . 6|. 1 .
7 . .|. 1 .|. . 3
. 2 .|4 . 3|. 7 .
. . .|5 . 2|. . .
. 6 5|. . .|4 8 .
2 9 .|. . .|. 6 5
| 9      5      1247  | 1267   2467   147   | 1267   3      8     |
| 18     3      6     | 127    279    179   | 5      4      1279  |
| 14     147    1247  | 8      234679 5     | 12679  29     12679 |
| 345    8      34    | 9      257    6     | 2      1      24    |
| 7      4      49    | 2      1      8     | 2689   259    3     |
| 156    2      19    | 4      58     3     | 689    7      69    |
| 1348   147    13478 | 5      346789 2     | 1379   9      179   |
| 13     6      5     | 137    379    179   | 4      8      1279  |
| 2      9      13478 | 137    3478   1478  | 137    6      5     |

To open the Daily Nightmare from this website, use File/WWW/Daily Nightmare to load it. Needless to say that you need an Internet connection to use this input method. When the connection fails, you will see an error message. The daily Sudoku competition at can also be found in the WWW submenu. As long as Mike Mepham does not change the lay-out of the web page that contains this puzzle, you will be able to load this interesting puzzle straight into SudoCue. When the page changes, you may need a newer version of the program. The News page of this site will tell you when this happens.

The last method is not really an input method, but certainly a way to load interesting puzzles into the program. Use the internal generator to create a random puzzle. Select File/New Random from the menu and choose one of the 5 custom defined difficulty levels. The key combination Control+Shift+1 through 5 can be used as a shortcut.

Each of these input methods always validates and rates the puzzle. A message will show for incomplete puzzles, allowing you to continue with the solver disabled.

Custom Properties

SudoCue allows you to save some custom properties in the puzzle file. Only the .sdk file format supports this feature. Select File/Properties from the menu to open the Properties dialog. Besides other useful properties, you can copy an URL from your browser and paste it as a hyperlink by pressing the Paste URL button. The hyperlink can be activated straight from the Properties dialog. Use the Save function to save the changes you made to the properties.

Print Sudoku

Select File/Print from the menu to open a print dialog, which allows you to print the current sudoku. This feature can be used when you are away from your computer and still want to play sudoku. Use a pen or pencil to complete the puzzle on paper. For difficult puzzles, you may also want to use an eraser.

New Print functions will be added in the next release.

Navigating a Collection

On this website, I often publish sudoku collections in .sdm format. When you open such a file, only the first puzzle is shown. Use the menu options File/Next and File/Previous or the Page Down and Page Up keys to navigate through the collection. The status bar will show the size and current position in the collection in a separate panel. If you want to jump to a specific position, click this panel in the status bar and you can enter a new position in a popup dialog.

Rating a Collection

There is a menu option File/Classify that can be used to mass-rate puzzle collections. When you select it, there will be a save dialog that allows you to name the .scl file to which the ratings will be written. For large collections with many extremely difficult puzzles, rating a collection will take a long time. The details for this format are explained in the File Formats document.

Copy Puzzles to the Clipboard

When you communicate about sudoku puzzles with fellow players, you must be able to exchange the puzzle or event the current state of solving. The various copy options in SudoCue allow you to copy various aspects of the puzzle to the clipboard, to be pasted into another program or on a website.
Here is a list of all copy variations:

Menu optionKeyboardDescription
Copy CluesControl+CCopies the original puzzle to formatted text.
Copy FilledControl+FCopies the puzzle with all placed digits to formatted text.
Copy MarksControl+MCopies the candidate grid to formatted text. The format is determined by an option.
Copy Solution Copies the solution to an 81 digit text string.
Copy Merged Copies the original puzzle merged with the solution to an 81 character text string. The original clues are represented by digits 1 through 9, the solution by letters A through I. This is a compact format which I use for storing large amounts of puzzles in a database, saving half the disk space.
 Shift+F12Copies the grid without the cursor to an image format. Paste into an imaging program.

Copy Marks will ignore the formatting options because it uses its own formatting. The formatting options can influence the output of the first 2 items in the list. Select Tools/Options from the menu and click the Copy tab. There are 4 independent formatting options, and 2 buttons to select the most commonly used settings.

Alternating Pens

In Play mode, you can use two different colored pens. Normally you would only use pen 1, but when you reach a point where you are not certain how to proceed, you can use the second pen to try a few alternatives. At some point, you want to make a decision. When the chosen direction is not correct, use Edit/Erase Pen 2 to remove all your entries with this pen. When you are certain you’re on the right track, use Edit/Confirm Pen 2 to rewrite all pen 2 entries with pen 1. Now you can continue with pen 2 or proceed with pen 1. This is a typical commit/rollback way of working. You can change the standard pen colors in the options dialog. The status bar shows which pen it currently active. As an extra reminder, the dotted focus rectangle also uses the color of the selected pen.

Undo and Redo

For any individual move, you can use Edit/Undo to revert the last move. A Redo function allows you to change your mind once more. Undo and Redo operate on the following moves:

  • Place a digit
  • Clear a digit
  • Clear a pencilmark
  • Set or restore a pencilmark
  • Solver Step

Mass moves like confirming or erasing Pen 2 or completely solving the puzzle cannot be undone. The Restart option is the only alternative.

Restart or Recalculate Marks

When you have messed up a sudoku and you cannot repair the damage, you can select Edit/Restart from the menu. The keyboard shortcut is the Home key. Restart will remove all digits placed in play mode. The given digit remain in their cells. Pencilmarks are restored to their initial setting.

When restoring the pencilmarks is your only concern, you can use Tools/Recalculate Marks as an alternative. This leaves the placed digits unchanged, but it will reset the pencilmarks to match these placed digits.

The Filter Buttons

On top of the main program window you see a line of filter buttons. Many of the tools provided by SudoCue require filtering the data for a specific digit. The filter buttons can help you here. In the options dialog, you can choose which tool must be activated when you click a filter button. This default will only be activated when no other filtered tool is active, otherwise the filter will only select another digit for the active tool.

The selected filter button(s) will be shown in a pressed state. When you click a pressed filter button, it will be turned off. Regardless of the filtering function, the numbers on the buttons are also used to show the state of completeness for these digits. When all instances of a digit have been placed, the digit lights up in green. For digits with 8 instances placed, the digit is shown in yellow. You have a Last Digit move waiting. In all other cases the digit has a black color.

You will often want to cycle through the digits with a tool active, so you can inspect the patterns for each digit. Use the comma and period keys to navigate left and right, respectively. As a reminder, these keys have the < and > symbols showing you the right direction.

Filter buttons are automatically selected by the program when a hint is shown, but only when the Watch tool is active.

The LEDs

The LEDs below the grid show which candidates are available for the cell that has the focus. The colors for both states can be set in the options dialog. The LEDs are only enabled when a tool is active that shows the candidates. These are Markup, Ultra(color), Links and Peers.

Users of an earlier version may remember that the LEDs were also used for tool filters. The filter button have replaced this function.

The Status Bar

For a quick check of the current state of the program, you should take a glance at the status bar at the bottom of the main program window. When you hover above the panels, a tooltip will come up saying what it is you’re looking at.

VariationThe sudoku variation currently selected.
Program modeThis can be Setup, Pen 1 or Pen 2. These modes have been explained earlier in this guide.
Selected ToolThis is slightly redundant, because you can already see the effects of the tool in the grid.
When Markup is used in combination with another tool, only the other tool is shown in this panel.
Empty panelThe file name has been moved to the title bar, so here is a little room for expansion.
Position panelOnly used when a collection file is open. It shows the position in the collection, followed by the size.
The panel has no tooltip, but it is the only panel that responds to a mouse click.
Filled cellsHow many cells contain a digit. You can count this yourself, but what do we have computers for?
Solved cellsWhen this is less than 81, the puzzle cannot be solved completely, and must have multiple solutions, or you should enable a few more solving techniques. When it reads --, the puzzle is inconsistent and cannot be solved at all.
Difficulty ratingThis number is calculated by adding the base scores for all solving steps, with some charges and discounts for special situations in the solving path. Details for this rating can be found in the Analysis Dialog. The tooltip will show the rating name that corresponds to this number.

The Tools

SudoCue is mainly used as a helper program. There are many people who do not appreciate any help from a computer program to solve a sudoku puzzle. Others could use a little help, but not so much that it would spoil their puzzling experience. There is also a group who use the helper for educational purposes. To learn how to look at the grid in different ways, so they can find new and quicker methods to solve a puzzle. There are also a few hint-hoppers.

To serve the needs of all these different people, there is a wide range of tools available. Remember, it is your personal choice to call these tools into action. You can solve the easy puzzles without any tools. When the going gets tough, you need more tools. When you have upgraded to the more difficult puzzles, it is most likely that you prefer to have the power tools active from the beginning, so you can get as soon as possible to the nitty gritty part of the puzzle.

Here are the tools in increasing order of intrusiveness:

Focus A very mild non-intrusive tool. By graying all digits other than the filtered digit, you are better able to focus your attention to the patterns for that digit. No additional information is revealed.
Hatch This tool used to be my favorite, but I hardly use it any more. You see, I’ve also upgraded. This tool is perfect for sudokus that can be solved with singles and locked pairs only. Most puzzles published in papers and puzzle books fall into this category. The hatch tool was implemented after I had read the Sudoku entry in WikiPedia. I liked the idea to include a tool that allowed you do cross-hatching as described in the article.
Watch This is similar to the filter tools in Simple Sudoku. You select a filter button and all cells that allow this digit as a candidate are highlighted with a different background color. This helps you recognize patterns that can be used in solving logic. The Watch tool is more intrusive than the Hatch tool, because it shows the candidate patterns, not the result of placed digits. I use this tool a lot myself, mainly because it helps me to quickly spot locked candidates. With this tool active, you can use the space bar to toggle candidates for the filtered digit on and off. With the arrow keys and the space bar, the effects of various patterns are quickly dealt with.
Color This tool shows the same candidates as the Watch tool, but with a neutral background color. You can color the selected cell using keys A, B, C, D, E, F and G. By using these keys in combination with the Shift key, an additional 7 colors can be selected, giving you a total of 14 different colors. In the options dialog, you can change these colors, but it is a good practice to pair the shifted color with the non-shifted color. Parity in chains can be visualized by alternating colors. The intrusiveness of the Color tool is similar to the Watch tool, since you have to apply the colors manually.
Peers This tool has been added as a non-intrusive alternative to full candidate markup. The tool draws all attention to the selected cell. Only the selected cell shows the remaining candidates, which you can toggle with the LEDs. All cells are dimmed, except the direct peers of the selected cell. If you have trouble spotting naked singles in the grid like I do, this tool helps you verify that a naked single does exist in the selected cell, without all naked singles staring you in the face because of the candidate marks. Advanced players have no use for this tool, as naked singles is not the fun part of their puzzling experience. Peers is a typical starter tool, although it is a tad more intrusive than the Focus tool.
Links This tool is for advanced players. The main purpose is to show the player where strong pairs are located in the grid, so the player can use these in advanced strategies like Multi-Coloring, XY-Chains or Forcing Chains. When you activate the tool, the grid will show small arrows pointing from the candidate marks in the direction of the second candidate in the strong pair. Each pair can be seen as two candidates pointing towards each other. The Links tool can be activated for a single digit, or for all digits together. The grid will look very busy with the Links tool active. As a bonus, all hidden singles have a small circle around them. As an advanced player, you should be ashamed when you see them like this. The LEDs are available when this tool is active.
Markup Also known as pencilmarks. The most commonly used solving tool. The tool shows which candidates are still valid in each cell, by showing them as little numbers or dots. The shape, size and configuration can be set in the options dialog. This is the only tool that you can choose to be active from the beginning. With the Markup tool active, the candidate LEDs below the grid are also enabled. Candidates can be toggled by clicking the appropriate LED, typing Control+number key, and you can also disable a candidate using the context menu, which is brought up by right-clicking a cell. The Markup tool is fully synchronized with the Watch and Color tools. You can even use Markup in combination with any of these 2 tools. It is a very intrusive tool, so you should avoid it when solving the easier puzzles.
It is also possible to enter your own pencilmarks manually. To do this, set the following on the Options screen: “Markers visible at the start of a new game” and “At the start of a new game, all candidates are disabled”. The LEDs will now allow you to enter pencilmarks.
Ultra Color The color variation of the Markup tool. It does all that the Markup tool does, but it also shows candidate colors. You may need to increase your cell size to make it easier to use. Use the same letters, optionally in combination with the Shift key, to select a color. This color shows in the lower right corner. Click any candidate to give it the selected color. Click again to remove the color from the candidate. The colors you place on a candidate are also shown as cell background color with the Color tool active. You can switch between these tools to alternate a single digit view with the broad overview showing the color patterns for all candidates. A very good tool to build implication chains.

This set of tools can help you solve any sudoku, even the most difficult ones. To disable all tools, use Tools/All Tools Off, or simply press the escape key. You can set a variety of tools options, to customize the program to your preferred way of solving.

Use Tools/Remove Colors to set all candidates back to neutral color. This affects the Color and Ultracolor tools.

Use Recalculate Candidates to restore all candidates that are allowed by Da Rule. Candidates removed with reduction techniques will be restored, but not those eliminated by placed digits.

Fast Cross-hatching

If you want to check the effects of a placed digit in a single cell, Shift+Click that cell to toggle the Hatch effects only for that cell. To activate the Hatch tool for all cells containing that digit, use Control+Click. This activated the Hatch tool and selects the filter button corresponding to the digit in the cell you clicked.

In the next release, these fast hatching methods must be used with the right mouse button. The left mouse button is used in combination with these keys to select multiple cells.

Multiple Digit Watch

It can be a daunting task to find naked and hidden subsets. Naked pairs are a breeze, but a hidden quad is a needle in a haystack type of operation. The Watch tool allows you to select multiple filter buttons.

Select a filter button, then Shift+Click a second button. This filters all cells that have both digits as a candidate. Shift+Click another filter button to look for 3 candidates. You can expand this set at will. When you Shift+Click a button that is already included, it will be excluded.

As an alternative, use Control+Click to filter all cells that have either of the digits as a candidate. When you add more digits, the last click determines if the AND or OR relationship will be used. This allows you to look at the same set in 2 ways, without having to select all buttons again.

The cycling keys can still be used when multiple filter buttons are selected. The program then cycles though all possible N digit combinations.

Integrated Tabling

With the Ultracolor tool active, you can use colors to build implication chains. It is also possible to let SudoCue take over this error prone task. Just Control+Click a candidate and the program will color all candidates that are forced to true or false by this assessment. The true condition uses the same color as you selected with keys A through G. The false condition uses a separate color, which can also be customized in the options.

Previous colors will automatically be cleared when you Control+Click a candidate. No longer do you need to clear colors via the menu. A quick inspection of the available candidates can now be performed. This is a very spoily tool, but ideal to find magic cells.

The Scrambler

Here is a nice exercise as an introduction. Take a valid sudoku puzzle. Move all givens in row 1 to row 2 and all givens in row 2 to row 1. Solve both the new puzzle and the original and compare the solutions. You will notice that rows 1 and 2 have been completely exchanged in the new puzzle. For your next trick, replace all clues for digit 1 with digit 2, and replaces all digits 2 with a 1. Solve both puzzles. In the solution, all digits 1 and 2 have been exchanged. This is the principle behind scrambling.

There are 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 = 362,880 ways to permutate the 9 digits.
There are 6 ways to permutate 3 rows or columns in a single block, giving 6 x 6 x 6 x 6 x 6 x 6 = 46,656 permutations.
There are 6 ways to permutate 3 blocks of rows or columns together, giving 6 x 6 = 36 permutations.
Finally, you can swap the rows with the columns by mirroring the entire grid on a diagonal.
As a result, there are 1,218,998,108,159 mathematically equivalent sudokus for each puzzle. That’s a lot of puzzles!

When you encounter an interesting sudoku puzzle, use the Scrambler to replace it with one of its mathematical equivalents. The puzzle will be unrecognizable, have a completely different solution, but requires exactly the same solving techniques as the original. As a side effect, you have created a completely new puzzle for which any copyrights claimed for the original no longer apply. You can use the Scrambler as many times as you want to create a whole series of equivalent puzzles. Symmetry of the original puzzle will be lost in the process.

The Optimizer

Here is another neat little tool that you will not find in any other sudoku program. Like before, we will start with a little exercise. Take a valid Sudoku puzzle. Remove a given number. Solve the reduced puzzle. Does it still have a unique solution? Then you have a non-minimal puzzle. If it has multiple solutions, replace the digits and remove another clue. Repeat this process until no more clues can be removed without leaving a puzzle with multiple solutions. The final result is a locally minimal puzzle.

If you followed my instructions, you have manually done what the Optimizer can automatically do for you. Create a locally minimal puzzle.

There are 5 alternative menu options for the Optimizer, from Keep it Easy to No limit. This will use your solver and rating settings, and not reduce the puzzle in such a way that it becomes too difficult for you. Only the No Limit setting guarantees that you will actually end up with a minimal puzzle.

The Optimizer will have little effect on puzzles generated by SudoCue or those found on this website. These are already symmetrically minimal. You will discover that many sudokus published in newspapers, magazines and puzzle books can be optimized in a substantial way. The result is always a harder, and probably a more interesting, puzzle.

The Solver

SudoCue has a solver that is essentially a “bag of tricks”. With the Options dialog, you can decide which tricks go into the bag and in which order they will be executed. Each time a trick is successful, the solver will reset and continue with the first trick in the bag. This makes certain that, from all the available tricks, the easiest is tried first. The resulting solving path may not be the shortest, but it will certainly be the one with a strong bias towards easier tricks. As a result, the puzzle ratings are more reliable.

The solver comes into action each time the puzzle is validated. This happens when you open a file, paste a sudoku from the clipboard, or when you switch to play mode after entering the clues in setup mode. You can also invoke the solver on demand. The information given to you by the program is often produced by the solver, as is the puzzle rating, the hints, the solver log and the detailed data in the Analyzer window. All this data therefore must be interpreted with the solver settings in mind. If you are wondering why the program gives you a complex hint, when you see an easy step, you should check in the settings that the easy trick is enabled and placed before the complex trick. This is the price you pay for the flexibility that the solver offers you.

There are two ways in which a trick takes effect. Either it results in the placement of a digit in a cell, or it causes one or more candidates to be eliminated. The placements are often caused by low level tricks, like naked and hidden singles. Only a few advanced tricks have a placement as the immediate result. Sometimes the solver will perform only eliminations when it has the possibility to do placements as well. This only happens in cases where the same trick could result in simultaneous placements and eliminations. Subsequent singles tricks will take care of the placements anyway.


Soon there will be a new definition in Webster’s Dictionary for the term “Hint Hopper”: A person who repeatedly presses F8 in SudoCue. When you select Solver/Hint or press F8, the program will check the solving path created by the solver. For each step, it will check whether all the placements and eliminations are present in the grid you are working on. If any effects are missing, the program will show you the step as a hint. A brief explanation is shown in the status bar. This temporarily hides the panels from the status bar. The cells that play a role in the step are colored with an outline. The following list shows the meaning of the colors:

Yellow When a single yellow cell is present, there is usually a placement that can be made in this cell. When multiple yellow cells are present, they highlight the cells that play a leading role in the hint.
Blue These cells play a supporting role in the hint. These are often the remaining cells in a house or the two houses that intersect, the cells that form a pattern, or which are otherwise used as supporting evidence. In many cases, the blue cells are not explicitly named in the hint.
Red These cells contain candidates that can be eliminated as a result of the step. This overrules the yellow outline, so a cell with a leading role will have a red outline when it contains candidates which will be eliminated. This is often the case with hidden subsets.

Besides the outlines, you will notice the following events happening in the grid: The focus is moved to the first cell that does not match the state it should have after executing the step. This means that you must either place a digit in that cell or remove one or more of its candidates. When the Watch tool is active, the program will switch to the digit that is influenced by the hint. This only happens for single digit techniques. Naked and hidden subsets will not change the filter, but singles, locked candidates and X-Wing will. Check the list of solving techniques in the options panel to see which techniques affect only single digits.

Any action that you take will clear the hint, even when you have not completed all placements and eliminations required. You can also clear the hint with the Escape key. Pressing Escape twice will first clear the hint and then disable all tools.


When you choose Solver/Step or press F11, the next solver step will be executed. This is a very lazy way to solve sudoku puzzles. The step solver does not show the outlines, but it will show the brief explanation on the status bar, and change the focus to the last cell that was modified by the step. Experienced solvers use this function to quickly skip the basic steps and fast forward to the more interesting parts of the puzzle.


When you are giving up on a puzzle and insist on seeing the solution, press F9 or choose Solver/Solve. Staring at the solution will not teach you anything and there is no honor in it, especially when you use the solution to participate in a sudoku competition. As a reminder, all placements made by the solver have a green color. When the solver has corrected one of your incorrect entries, it will be shown in red.

Solve From Here

This menu option does not change the grid, but instead it takes the grid in its current state, including all the placements and eliminations you made in play mode and restarts the solver with this data. Because this is only a partial solve, the rating figures are no longer reliable. Use this function to answer the frequently asked question: “What is the next step given the current state of this puzzle?”. When you have removed too many pencilmarks, or made an incorrect placement, the solver may fail to solve the puzzle from the given position. This is also very useful information.


The help index for all versions up to 1.4.1 will show you the crappy document that I’ve written earlier. As of version 1.5, Help/Index will lead you to this online document.

The About dialog will tell you who wrote the software, the current version and its release date, the version of the .Net FrameWork that is used by the program, a link to this website, and a button that allows you to check whether your current version is up to date. When a more recent version is available, a message box shows the most recent version number, the release date and a summary of the changes. You may encounter one of my sarcastic remarks when you perform this check with a very old version.

The Options Dialog

This dialog almost deserves a separate manual. Many players have developed their own personal style in time. The program recognizes this fact and allows you to change the settings in a way that your sudoku experience is best served. The options are saved in a personal configuration file. When the computer has multiple users, each user uses his or her own settings. These settings are not lost when you install a new release. Even when the program has added new options, they are automatically set to their default and used in combination with the options you saved in the previous version.

Helper Options

Markup and Watch: Automatically remove invalid candidates

Check this option when you prefer the program to remove pencilmarks for candidates that are invalid because a peer already contains that digit. This option saves you a lot of administrative work. However, if you are a “do-it-yourself” type of sudoku player, clear the checkmark for this option.

Markup and Watch: Markers visible at the start of a new game

Each time you start a new game, all the helper tools are turned off by the program. Players who have been using Simple Sudoku may have gotten used to having the pencilmarks visible at all times. By checking this option, the program will emulate this behaviour. This only affects the state of the Markup tool at the beginning of each game. During the game, you can turn the pencilmarks off and on whenever you like.

Markup and Watch: Show Markup in Watch and Color modes

When checked, this option will cause the Markup tool to stay active when you activate the Watch or Color tools. For many players, this is the preferred setting. Uncheck this option if you want the pencilpark hidden when you activate the Watch or Color tool. This is the way SudoCue used to work in earlier versions.

Markup and Watch: Show naked single in large font

When there is only a single candidate left in a cell, it is obvious that the digit must be placed there. To draw your attention to this easy scoring opportunity, check this option and the program will show the lone candidate in a large font. Leave this option unchecked if you prefer the challenge of locating singles amongst the small pencilmarks.

At the start of a new game, all candidates are

When you solve the daily puzzle in your newspaper, it does not come with all pencilmarks pre-printed. After you have done the elementary work, you will pencil in these marks yourself. This is a hands-on way of using pencilmarks. When you solve sudoku puzzles using a computer program, the program can take over this administrative task. Most programs start with all pencilmarks present, gradually removing them as you solve the cells and perform the necessary eliminations. “Whatever remains ... must be the truth”, good old Sherlock used to say. With this option, you have the choice to use pencilmarks in both ways. Choose Disabled when you want to add the pencilmarks manually. Choose Enabled when you want to start with all pencilmarks present.

Watch tool also colors: Cells containing given digits or placed digits

The Watch tool highlights the cells that allow candidates indicated by the filter buttons by giving them a different background color. To recognize patterns, you do not need to include the cells that already contain a value. Some players find it easier to see patterns when the placed digits are also highlighted. A program like Simple Sudoku does this, but only for cells that are not given as initial clues. These two options let you decide whether or not to highlight given digits and/or user digits in the Watch tool.

Default tool for filter buttons

With this option you can tell the program which of the four listed tools should be activated when you click a filter button. Select your most frequently used tool. For most players, that is probably the Watch tool.

Markup style

Choose the style of pencilmarks that works best for you. When you choose Dots, the program will draw little dots in a 3x3 pattern. This style obfuscates the digits behind the pencilmarks. It is also the best choice if you have selected a very small cell size, since the dots do take up less space than digits.
There are 3 alternative configurations for small numbers. A keypad similar to your phone, with 123 on top, the keypad you have on your computer keyboard with 789 on top, and a compact format. The candidates are written in 1 or 2 rows in ascending order in the middle of the cell. Because this style has the pencilmarks closely written together, you cannot use the Links tool in combination with this style. The tool is disabled when you choose this setting.

Double-Click and Insert key place

A quick way to proceed in the easy parts of a puzzle, especially towards the end, is to use the autofill feature of SudoCue. These two options allow you to select in which cases the autofill tool will place a digit in the selected cell when you double-click it, or press the Insert key. You can enable naked singles and hidden singles independently. When you disable both, you have effectively disabled the autofill feature.

Cell naming format

The naming of cells is a matter of personal taste. When you have started to use a certain naming convention, you most likely never switch to another system. Not all computer programs have adopted your naming convention, leaving you the task to translate the hints into your own “language”. This setting allows you to instruct the program to use your naming convention in hints, the solver log, and other communication towards you. To verify that you have made the right choice, check the rulers to the left and above the grid. They should show your chosen coordinates.

Anything you type in the format string will be inclided in the cell naming, but to make it a little meaningful, use the placeholders identified with a hash symbol. The character following this symbol determines what will be written in its place. The following table shows the possible placeholder characters. Please note that the characters are case sensitive.

rRow number, numbered 1 through 9 from top to bottom
RRow number, numbered 1 through 9 from bottom to top
iRow letter, named a through i from top to bottom
IRow letter, named A through I from top to bottom
jRow letter, named a through j from top to bottom (letter i is skipped to avoid confusion)
JRow letter, named A through J from top to bottom
cColumn number, numbered 1 through 9 from left to right
CColumn number, numbered 1 through 9 from right to left
yColumn letter, named a through i from left to right
YColumn letter, named A through I from left to right
zColumn letter, named a through j from left to right
ZColumn letter, named A through J from left to right
#Include the hash symbol

The drop-down list allows you to pick one of the most commonly used cell naming options, but you can also define your own format.

Solver Options

If you only used an earlier version of SudoCue, you will notice some serious changes in this department. The left side shows a list of all solving techniques that have been built into the solver. From this list, you can assemble your personal “bag of tricks” that suit your solving skills. As you learn new techniques, you can enable more tricks later on. Left-Click an item in the list and the dialog will show a brief description. There is also a link to the detailed explanation in the solving guide on this website. You need an Internet connection to use the link, but who doesn’t, nowadays?

The dialog also shows the type of technique. Placement vs. elimination, pattern vs. chains, single digit vs. multiple digits.

The base score is the number of points the solver adds to the rating when it encounters this trick. Tho program will also add some additional points depending on the search time. After calculating discounts for flow in the solving path, the final score for this step will be saved, which you can inspect in the Analyzer window.

The admin score is added to the total score the first time this technique is encountered, whether it is successful or not. The use of pencilmarks is a good example. At some point, you need to use pencilmarks. This will cost you time. After the pencilmarks are initially drawn, maintaining them is easy, so no additional score is added for that maintenance. It is possible that you do not need the pencilmarks after all, but trying a certain technique already forced you to place them, so the damage is done.

The last property you can set is the minimum difficulty level. This forces a puzzle to a certain difficulty level when the technique is required to solve it. You need to be careful with this setting. When you set it too high for a relatively simple technique, the generator may not be able to find puzzles that meet both the score ranges and the minimum difficulty level for the tricks it encounters. When the generator is taking a long time and fails to produce a good puzzle, you need to recalibrate your settings.

Click the selected solving technique to disable or enable it. Use the Move Up and Move Down buttons to reorder the tricks in your bag. Please note that the last two tricks (Brute Force and Surrender) are unmovable. They must always be the last in the list. It is also not possible to disable the Surrender step. The solver must use it when all other techniques fail.

There are a few preset buttons to make a quick selection. Select All does not need an explanation. Restore Defaults enables a set of techniques for the average player. No Uniqueness disables the techniques that presume a unique solution. Some players feel uncomfortable using these techniques. I hope you are not one of them, because they are amongst the cleverest tricks that I have encountered in the world of Sudoku.

Generator Options

The left side shows a list of symmetries that are recognized by SudoCue. Select your personal favorite and the generator will obey this symmetry requirement when creating random puzzles. Choose Random symmetry if you like to be surprised. Choose No symmetry when you no not appreciate the aestetic value of a symmetrical sudoku. Your puzzles will be absolutely minimal and slightly more difficult when generated without symmetry.

The right side offers you the opportunity to define your own difficulty levels. Most sudoku publishers use 4 or 5 different levels. The fifth level has no threshold. It automatically contains all puzzles the exceed the limit for the fourth level. All other levels can be redefined by entering a new limit. Do not make the mistake of giving a higher level a lower limit. The generator will not be able to find puzzles matching that level. The names you enter will be used in the main menu to identify the levels.

You can set the behaviour of the generator in case it cannot find a puzzle with the desired properties. The generator examines random puzzles, so there is a possibility that it cannot find one you like. You can set the maximum number of attempts made by the generator. After that, the generator will try a lower difficulty level when you have set the option for it. This ensures that the maximum effort will be put into presenting you with a puzzle. If all attempts fail, you will receive an error message.

Check the option to keep symmetry intact for the Optimizer, when you do not want it to destroy the current symmetry. For non-symmetrical puzzles, this option has no added effect. The result is a symmetrically minimal puzzle, where no clue can be removed without either breaking symmetry or resulting in a puzzle with multiple solutions. When you uncheck this option, the Optimizer will go all the way and forget about symmetry.

Color Options

This panel allows you to change the colors used in the grid and by the tools. Click the square showing the color to change it in the Color dialog. You can choose a separate background color for a given digit, so it stands out against user digits. When the Watch or Color tool is active, this difference is temporarily disabled, so does not distract you from the more meaningful colors in the grid.

Grid Options

On this panel you can change the remaining properties that affect the appearance of the main program window. You can independently change the width and height of the cells, but most players prefer squares. Because there is a limit to the maximum font size, an extremely large grid does not increase readability. Instead, it increases the chance of screen flickering, because it takes more time to redraw the grid. A minimum cell size of 25 x 25 pixels is recommended if you do not use pencilmarks. This gives you about the size of a small sudoku published in a newspaper. A minimum of 32 x 32 is recommended when you use pencilmarks. Use a minimum size of 40 x 40 when the Links tool is used.

When you change the font, you should verify that the font is not too small for readability and not too large for the available space. Also check that the candidates work fine with the Links and Ultracolor tools.

LEDs Options

This panel lets you choose the colors for the LEDs below the grid. There are two colors to be set, one for the available candidates and one for the eliminated candidates. The default colors green and red work fine for me.

Copy Options

The options in this panel help you customize the formatting of sudoku grids which are copied to the clipboard with the Edit/Copy Clues and Edit/Copy Filled menu options.

New line after each row

Check this option to insert a Carriage Return and Line feed after each row. Uncheck it to copy the puzzle to a single line of data.

Wide spacing between cells

Check this option to insert a blank before and after each cell position. This makes the puzzle easier to read by humans. You should use this in combination with “New line after each row” and “Draw box outlines”.

Draw box outlines

Check this option to insert characters -, | and + to draw rough outlines for the 3x3 boxes. You should use this in combination with “New line after each row”. When you uncheck this option, the digits are not separated. This is a more compact grid representation.

Use dots for empty cells

Check this option to show the empty cells with a dot. This improves readability, as you can better see where the clues are. When you leave this option unchecked, the empty cells are represented by a zero.

Click the All On or All Off buttons to set one of the two most popular settings.

Copy marks with braces around sets of candidates

When checked, the candidate grid will be copied in a format that encloses the candidates for each cell in curly brackets. This format is required by some other sudoku program. Leave this option unchecked if you want to keep the posh formatting of the candidate grid.

Extra Windows

The main window of SudoCue does not show all the bells and whistles that the program offers. It only shows the puzzle and the tools that you may need to solve it. The vast majority of players do not need all the technical details that can be retrieved from the puzzle. They just want to play Sudoku. For the die-hard players, the extra windows can be opened to show details that you need for a more thorough analysis of the puzzle at hand. Several additional windows provide specific information about the rating, the solving path, the candidate status and the data required for specialized solving strategies. Only a few players will feel the need to use these special features. All the extra windows can be kept open during play and will automatically be updated as required.

The Analyzer Window

Use the menu option View/Analyzer or press F7 to open the Analyzer window. In this window you will find a report created by the solver, which provides more insight in the difficulty rating and the solving techniques required to solve the sudoku which is currently loaded.

On the left top, you see a summary of solving techniques used in the solving path, each followed by the number of times this technique has been successfully applied. A long list will tell you that you have an interesting puzzle loaded, with many different tricks that you can use. The Daily Nightmare is one of the few sudokus that manages to fill this list to the point that the scrollbar is required.

Below this summary you will find the number of solving rounds (or steps, if you like), the total score with the difficulty level that matches this score, and the type of symmetry found in the clues.

The right part of the Analyzer shows the individual solver steps, with some numerical data. This data consists of the following items:

  • Solver step number
  • Number of cells to be solved
  • Number of candidates remaining
  • Number of alternatives to the technique found (temporarily disabled)
  • Number of points added to the total score
  • Name of the technique that was used

The analyzer is automatically refreshed when you load a new puzzle or use the “Solve From Here” menu option.

The Log Window

When you want to know how the difficulty of the sudoku has been determined, you should use the Analyzer. When you want to know every little detail about the way the solver has dealt with the sudoku, use the Log. Use the menu option View/Log or press F6 to open the Log window.

The main body of this window shows the solver log. Initially the log is fully expanded, but you can collapse individual parts of the log to focus on the parts that you are interested in. Use the filter selectors on top to make a selection for a specific digits, row, or column that you want to know more about. When you left-click a line in the log, the lower part of the Log window shows a more detailed description of the log entry. When the line contains a solver step, the relevant cells in the main window will be outlined in the same way as when you ask for a hint by pressing F8. Frankly, you are looking at the source of the hints.

The Copy button copies the content of the log to the clipboard, so you can paste it in a text editor, or post it on a sudoku forum. When you close the Log window, the highlighting in the main window will be cleared.

The Dancing Links Visualizer

This window is a little gimmick to show you how the Dancing Links algorithm operates. In SudoCue, the human style solver is fully integrated with the linked structures required for DLX. Use the menu option View/DLX open the DLX Visualizer. The window contains a 324 columns for each of the sudoku constraints. When the solver progresses, you will notice that the size of each of these columns decreases. When a constraint is fully satisfied, i.e. when only one candidate remains, the color changes to green. You will not see much dancing activity for the average sudoku, because the logical solving techniques usually take care of it. Disable the advanced solving techniques and load an extremely difficult sudoku. Now you will see the DLX solver in action. This would be my favorite screensaver...

The Status Window

This window is also more a gimmick than a user tool. Use the menu option View/Status open it. You will see a small window in the corner of your screen with a lot of numbers on it. The data is shown for each of the 9 digits, and there is also a total for all digits.

Cells filled is self evident and does not require an explanation.

Effective blocks counts the number of candidates that are eliminated because either the cell contains another value, or because another cell in the same row, column or box already contains that number. Each eliminated candidate is counted once.

Redundant blocks shows how many duplicate obstructions are present. When a candidate is already eliminated because another value is placed in the cell, another cell in the same row, column or box are redundant. For initial puzzles, you can see how efficient the clues have been placed in the puzzle. A low redundancy count is a sign of a cleverly constructed puzzle.

Candidates shows how many candidates are left after the eliminations are made. Initially, there are 729 candidates. (81 x 9). Subtract the cells filled and the effective blocks and you have the number of remaining candidates.

Groups filled shows how many houses (rows, columns and boxes contain clues or solved cells). This is equal to cells filled x 3.

The remaining lines show a break down of te number of remaining candidates for each digit in all houses. When there are groups with only one candidate, there are hidden singles present. The first column does not give a total, but shows the breakdown in number of candidates for the cells. When there the first number is greater than zero, there are naked singles in the grid.

This last feature can help you to determine what to look for in the grid, when you are having trouble finding hidden and naked singles, but you don’t want your fun to be spoiled by a hint.

The Braid Analysis Window

Braid analysis is a solving strategy that analyzes the distribution pattern of candidates in a 3 x 9 section of the grid. This can be either a vertical section (which we call a Tower) or a horizontal section (a Floor). The strategy was first introduced by Timothy Hamilton, and later developed by David P Bird. Use the menu option View/Braid Analysis open the window.

In the left bottom corner you can select any of the 3 towers or floors as the subject of your analysis. For readability, you work with a reflection of the tower over the anti-diagonal. This reflection is chosen, so that the traveling directions are not altered.

The grid shows the 27 cells organized in 3 boxes with their remaining candidates. Below the grid are the candidates that fit into each of the 6 possible traveling paths. The purpose of this guide is not to explain the solving techniques, so you should read the forum discussion following the link above if you’re interested in learning this technique.

The little matrix in the left top shows the summary. Candidates shown with a bold font are mandatory in that traveling path. This allows you to eliminate candidates that lie outside that path. More advanced analysis methods can be found in the forum discussion.

Your input is appreciated.

As of release 1.5.0, this user guide has replaced the help files that have previously been supplied with the program. The aim of this guide is to be very comprehensive and help you use the program to its full extent. When you encounter parts that need clarification, errors or if you like to see additional topics included in this guide, please contact me via email or the user forum. Your feedback is important to me. Even though it is freeware, I'd like to claim to be the author of the best sudoku program available. You can help me achieve that goal with your suggestions for improvement. The project is still very much alive, like this website. Thanks in advance, Ruud.
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