Sudoku

Forcing chain practice

A forcing chain tests a candidate by supposing it is true. Place it in your head, then follow the singles it creates across the board. If the trial runs into a contradiction, such as a cell with no number left, the candidate was false and can be crossed off.

This is proof, not guessing: you only act when the trial breaks the grid.

Suppose the blue cell is 2. That forces the yellow cells one after another, until the red cell has no number left. So the blue cell is not 2.

How to spot a Forcing chain

Start from cells with two candidates, where a wrong choice shows itself fastest. The hint for this step names the number it tried, how many cells that forced, and where the grid broke.

Practise it on this page

The board above deals positions where the Forcing chain is the only way forward, with every pencil note filled in. Cross off the candidates it rules out and press Next. Press Hint to see the pattern marked on the board.

In a full game, Extreme is the first level that can ask for it.