The playing field is a square of 9x9, divided into smaller squares with sides of 3 cells. Thus, all the playing field consists of 81 cells. They are already at the beginning of the game are some numbers (from 1 to 9), because the playing field blank does not make sense. Depending on how many cells are already filled, the concrete puzzle can be attributed to easy or complicated. In the puzzle has only one rule. You must fill free squares with numbers from 1 to 9 so that every row, every column and every 3x3 small square each digit would be met only once. How many cells are blank, the complexity depends on the game. Some puzzles can be solved in a few minutes, others can spend hours.