Mirrored from **Sudopedia**, the Free Sudoku Reference Guide

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:

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.

This page was last modified 15:07, 29 October 2006.