Rules
Sudoku rules
Sudoku rules are simple, which is why the puzzle is so flexible. The challenge is not learning many rules; it is applying the same few rules carefully.
The grid
A standard Sudoku grid has 81 squares arranged in 9 rows and 9 columns. It is also divided into nine smaller 3 by 3 boxes.
The heavier borders on the board show those boxes. Every square belongs to one row, one column, and one box.
The digits
Only the digits 1 through 9 are used. You do not add, subtract, or repeat a digit to make a total.
A completed row, column, or box contains each digit exactly once.
Rows, columns, and boxes
Each row must contain the digits 1 through 9. Each column must contain the digits 1 through 9. Each 3 by 3 box must contain the digits 1 through 9.
If a digit already appears in one of those three places, it cannot be placed in the square you are checking.
Givens and guessing
The starting digits are givens. They are part of the puzzle and should not be changed.
A well-formed Sudoku has one solution and can be solved by logic. If you feel forced to guess, pause and re-check the candidates around the square.