Sudoku HowTo

# Sudoku Rules

A standard Sudoku Puzzle is a 9 by 9 grid of cells or squares. Each cell will eventually contain a single value. I have used the term "cell" here to refer to the container for each value. A Sudoku grid of cells is further divided into units or regions. I have used the term "unit" on this site to refer to these groups of cells.
Units are rows, columns or boxes. A "row" is a horizontal line of 9 cells. A "column" is a vertical line of 9 cells. A "box" which usually has darker lines around it in published puzzles, is a 3 x 3 area of cells. Other names have been used for these units in the Sudoku world. I have used these names because they appear to be the most consistent.

A puzzle starts out with some values already filled in. These are referred to as the "given values" or "givens". Using these values and the basic Sudoku Rules it is possible to determine where to place all the other values.

The Basic Rule of Sudoku - Each unit (row, column or box) has to contain all the values, 1 to 9.

This also means that each unit will only ever contain one instance of any value. When you have 9 cells and 9 values have to be used, you cannot use a value twice.

Solving a Sudoku puzzle is the process of finding a cell that can contain only one value or finding a value that can only go into one cell of a unit.

Most puzzles published in Sudoku books or newspapers can be solved using the 3 or 4 simpler solving techniques described on this site. The simpler techniques are at the top of the strategy list. It is only the extreme puzzles that need the more complicated techniques.

If you are new to Sudoku start out with some simple or moderate puzzles and practice the simpler techniques. Get familiar with cross hatching and counting. These techniques in their basic form use the values already placed in a puzzle and do not require candidates for any cell be tracked.

Once you have mastered these, move onto the more difficult puzzles to practice box claims and row column claims. You can start tracking candidates and looking for naked pairs or hidden pairs. These techniques will solve a large portion of the Sudoku puzzles printed in puzzle books.

Only extreme puzzles will require the more complicated solving techniques. All of these techniques require the tracking of candidates for each cell. Some of the more complicated techniques get very difficult to identify when using pencil and paper.

If you have been working on Sudoku for a while and are here to learn more solving techniques try some of the sample puzzles or test yourself on the more difficult puzzles. Enable just some of the solving techniques so you can see how they work.

When you use this program to create puzzles it will create a puzzle of random difficulty. If you are trying to learn a particular technique or want easier puzzles disable some of the solving strategies. If you create a puzzle on this site while some solving strategies are disabled then the created puzzle will be able to be completed using only those strategies that are enabled along with Cross Hatching. The possible strategies in the right hand column are ordered by my perception of the difficulty to use the strategy.

Not every technique will work in every puzzle. You will need to experiment a bit to identify the signs that a particular solving technique might produce results in any given puzzle.

Once you have mastered the standard 9 x 9 Sudoku puzzles see if you can head around the same process using a 16 x 16 grid. These puzzles use the same logic but have 16 possible tokens for each cell and 16 of each type of unit.

Have Fun

2 March 2024
by Jacquie
RE: Comments and feedback for basicRules

I still don't understand why some squares are issued one of 2 colors in a chain of pairs.
27 June 2023
by John
RE: Comments and feedback for basicRules

Hello Steve. I think the solver is working as I intended.
If you only enable only one strategy the create process will find a puzzle that requires the enabled strategy at least once to solve.
If you enable multiple strategies, the create process will find a puzzle that only requires the enabled strategies to solved.
If you enable all strategies the create process will return the first puzzle it can solve.
The solver actually create new puzzle all the time and the difficulty of the puzzle is totally random.
If you would like to solve more advanced puzzles, but do not want to do forcing chains, then disable forcing chains and create the puzzle. Click Analyze to see how difficult it will be to solve. If the puzzle is easier than you want create a new puzzle.
I hope this helps you
27 June 2023
by Steve Garret
RE: Comments and feedback for basicRules

Great site! One great feature of the program is not working properly. Once I disable even one strategy then create a puzzle, the new puzzle is an "Easy" puzzle 99% of the time. I can no longer play an advanced game with the "Forcing Chain" strategy disabled. I am hoping you will fix this because I really enjoy your site. Thanks
9 February 2023
by lost
RE: Comments and feedback for basicRules

plugged in numbers from a partially solved puzzle and hoped to get next step. Not getting any yellow box with numbers, only green box.
9 February 2023
by lost
RE: Comments and feedback for basicRules

plugged in numbers from a partially solved puzzle and hoped to get next step. Not getting any yellow box with numbers, only green box.
3 November 2021
by John
RE: Comments and feedback for basicRules

Not something I considered when I built the site but if you use the Email Puzzle button and save that link somewhere then you can restart the puzzle from that point by pasting that link into your browser.
3 November 2021
RE: Comments and feedback for basicRules

Great site, many thanks for your great work. Please how do I save a puzzle I am working on in your site.
2 February 2021
by John
RE: Comments and feedback for basicRules

When you click on a cell and get the number entry boxes open there is a blue and green dot below the cell. Clicking in this dot will color the cell for you
2 February 2021
by Mike
RE: Comments and feedback for basicRules

With simple colouring in mind does the program allow colouring of specific cells? Great program by the way. Thank you very much!
7 November 2019
by John
RE: Comments and feedback for basicRules

At this time the program does have a way to do that. Sorry
14 March 2019
by Peter Livesey
RE: Comments and feedback for basicRules

How do I remove an error when entering the initial numbers for a puzzle?
30 July 2015
by Tony Hughes
RE: Comments and feedback for basicRules

What is the answer to the Qs on 9/7/14 ?
20 September 2014
by Arlette
RE: Comments and feedback for basicRules

Thank you so much for this site. It is a lot easier to understand than some of the other sites when it comes to more complicated techniques. I still have a long way to go but you are certainly helping. Keep up the good work.
9 July 2014
by John
RE: Comments and feedback for basicRules

It looks like I have a logic flaw here. If a number is placed in a cell while entering a puzzle, it can be changed to a different number by clicking on that cell again, but at this time it can not be removed altogether.
9 July 2014
by David Steward
RE: Comments and feedback for basicRules

Thanks for the program.

How do I remove an error when entering the initial numbers for a puzzle from a magazine?
13 January 2013
by Helen Eichman
RE: Comments and feedback for basicRules

I love this site! It really helps to know the next step in solving a puzzle even though some techniques elude me. Hopefully I can catch on using your information. Thanks!

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Sudoku Rules and Techniques

Sudoku Rules

Acknowledgments

I would like to acknowledge these sites referenced for strategy ideas and solving techniques.

SudokuWiki.Org by Andrew Stuart
PaulsPages.co.uk by Paul Stevens