Sudoku

XY-Wing practice

An XY-Wing is a pattern of three cells with two candidates each. The pivot holds two numbers, call them X and Y. One wing shares a row, column or box with the pivot and holds X and Z. The other wing also sees the pivot and holds Y and Z.

Whichever number the pivot takes, one of the wings is forced to be Z. So any cell that sees both wings cannot be Z, and Z can be crossed off it.

The pivot (blue) holds 1 and 2. Whichever it turns out to be, one of the wings (yellow) becomes 3. The red cell sees both wings, so it cannot be 3.

How to spot a XY-Wing

Look for a cell with two candidates that sees two other two-candidate cells, each sharing one of its numbers. Check that the two wings have their second number in common. The wings do not need to see each other.

Practise it on this page

The board above deals positions where the XY-Wing 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, Expert is the first level that can ask for it.