Backdoor

A backdoor is a placement of one or more candidates in their respective cells, which allows the player to complete the Sudoku with relatively simple solving techniques.

A popular alias for backdoor is magic cell. The backdoor digits are taken from the solution.

Each backdoor has two properties:

Size

A magic cell is a backdoor of size 1. A backdoor of size N requires the placement of N digits.

Technique set

The set of solving techniques that is required to solve the remaining puzzle after the backdoor digits are placed. There are two sets most commonly used for backdoor determination:

• Singles
• Simple Sudoku Technique Set (SSTS)

Backdoors are used to determine the difficulty of a Sudoku puzzle. The number of backdoors has an effect on the chance that a player can complete the puzzle by making a guess, rather than using logic.