The rules of Sudoku

Short version

You have to fill the rows and columns of the 9×9 grid in a way that each digit from 1 to 9 can only appear once in every row and in every column. In every 3×3 subgrid of the board each digit can appear only once.

Longer version

Sudoku is an excellent logic-based, combinatorial number-placement puzzle. The goal is to fill a 9×9 grid with digits in such a way that each column, each row and each 3×3 subgrid that compose the table contain all of the digits from 1 to 9. At the beginning you start with a partially completed grid, which generally has only one solution. The completed game is always like a Latin square, with an additional constraint about the contents of the individual regions. For example, a certain number can’t appear more than once in the same row, column or in any of the 9 3×3 subgrids. The game was popularized in 1986 by a Japanese puzzle company called Nikoli, under the name ‘Sudoku’, which means ‘digit’. Since 2005, it has enjoyed huge popularity all over the world.

The name Sudoku, meaning "single number"