Rules for solving sudoku puzzles.

Sudoku has 5 rules that must be followed in order to complete a board.

1. Each square must contain one and only one number.
2. Each row must contain no duplicate numbers.
3. Each column must contain no duplicate numbers.
4. Each section of the board must contain no duplicate numbers.
5. Only the number 1 to N can be used, where N is the size of the board.

On a 9x9 board, this essentially comes down to every row, column and board section containing the numbers 1 to 9, with no duplicates in either. An example of a complete board is shown below.

Examples of Sudoku Boards

Incorrect Sudoku Boards

Below is an example of a sudoku board that breaks some of the rules shown above

Note that the player has placed a number 6 in the 4th row of the 6th column. This breaks rule 2, 3 and rule 4. Note that the 6th column already has a number 6 in it (highlighted in red), this breaks rule 3. There is also a 6 already in row 4, which is also highlighted in red. This breaks rule 2. The 6 in row 4 is also in the same section as the grey 6, this breaks rule 4.

Correct Sudoku Boards

Below is an example of a correct board.

Here the number 4 is needed in the 6th column, as it already contains all the other numbers. The placement of the number 4 does not break any of the other rules.