Testing for Contradiction

Level: Advanced

Assume a value for one cell; if it breaks a rule, confirm the opposite.

When no more confirmations are visible, assume that one cell is filled or empty and follow the consequences. If the assumption makes some line's clue impossible to satisfy, then the assumption is wrong, so the opposite value is confirmed.

A good puzzle is designed to be solvable without such assumptions, but in practice quickly checking one cell for a contradiction can break through a deadlock.

Make assumptions one cell at a time, and in a way that is easy to undo. Once a contradiction is confirmed, lock that cell in and return to the basic techniques.

Related techniques