Let us define a Deadly Pattern as a configuration of candidates that leads to several solutions for a particular puzzle. The simplest example being a puzzle with only two unsolved Cells containing the same two candidates: such a puzzle has two solutions. A proper Sudoku puzzle has a unique solution, thus solving a puzzle should not lead to a "Deadly Pattern".

When you arrive at a situation where all unsolved Cells are "Bi-Value" Cells except one that still contains three candidates, consider the candidates in that Cell. If eliminating one of them leads to a Deadly Pattern (the two other candidates occurring twice in the Row and twice in the Column and twice in the Square to which the Cell belongs), then this candidate must be the solution for that Cell.

Indeed, keeping this candidate is the ony way to avoid a "Deadly Pattern".

In this example if candidate 8 were eliminated in F5, there would be two possible solutions:

• F1=1, F5=6, F8=8, D7=6, D5=8, E5=1, E2=8, etc
• F1=8, F5=1, F8=6, D7=8, D5=6, E5=1, E2=1, etc

With candidate 8 as the solution in F5, the puzzle has a unique solution.

You can practice this strategy by installing the SudokuCoach application on your Android™ device.