Rules of the solution of Sudoku puzzles

Although solving a Sudoku can be quite difficult, the Sudoku rules are quite simple.  A standard Sudoku is a grid of nine by nine boxes, further divided into nine three by three blocks. See the example below:

Basic rules for solving a Sudoku

In the Sudoku puzzle, the numbers from 1 to 9 must be filled in at the following way: 

• in each horizontal row each number can only appear one time,
• In each vertical column each number can only appear one time as well,
• In each block (shown here in brown) the same rule counts.

The name of the puzzle, Sudoku, is a Japanes word. The best translation is: "single numbers".

Contrary to what you might initially expect, solving a Sudoku can be done without mathematical knowledge. Logical insight is important to solve this kind of puzzle.

It is also useful to know that a puzzle always has only 1 solution. So if all fields are filled in and a 1 to 9 has been entered in every row, column and area, it is correctly filled in. In a Sudoku, a number of digits have therefore always been entered and the challenge for the puzzler is to fill in the missing digits in the manner mentioned above. The fewer numbers, the more difficult it is to solve the Sudoku.

Fill in the numbers with a pencil. As the Sudokus get more difficult you will notice that the rules above are not always enough to solve the puzzle. In that case, you will first have to fill in the numbers that could possibly appear in each box. If you see a possibility, pencil it in one corner of the little cubicle. You may have written 3 or 4 provisional numbers in pencil in the corners while solving the puzzle. While you're at it, you may notice that certain boxes only contain 1 number.

• You can probably already fill in that number definitively.
• Do not forget to check your logic behind each box you fill in; a single mistake can make the whole puzzle go wrong.