The playing field is a square of 9 × 9, divided into smaller squares with a side of 3 cells. Thus, the entire playing field consists of 81 cells. They already at the beginning of the game are some numbers (from 1 to 9), called tips. From the player it is required to fill the free cells with numbers from 1 to 9 so that in each row, in each column and in each small 3 × 3 square, each digit would occur only once.

The complexity of Sudoku depends on the number of cells initially filled and on the methods that need to be applied to solve it. The simplest are solved deductively: there is always at least one cell where only one number is suitable. Some puzzles can be solved in a few minutes, others can spend hours.

A properly composed puzzle has only one solution. Nevertheless, on some sites on the Internet under the guise of complicated puzzles, the user is offered sudoku variants with several solution options, as well as with branches of the solution itself.