A popular alias for backdoor is magic cell. The backdoor digits are taken from the solution.
Each backdoor has two properties:
- 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:
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.