X-Wing

Level: first needed in Extreme puzzles. In one line: A number fits in two rows only in the same two columns, so it is removed from the rest of those columns.

An X-Wing uses one number across two rows. If, in each of two rows, the number can only go in the same two columns, those four cells form the corners of a rectangle. The number must be in two opposite corners: one in each row, and therefore one in each of the two columns.

So both columns already have their copy of the number somewhere in the rectangle, and it can be removed from every other cell of those two columns. The same works with columns and rows swapped.

Example from a real puzzle

This position comes from an Extreme puzzle in Sudoku Arena, at the exact moment the X-Wing is the next logical move. Cells are named by row and column: R3C4 means row 3, column 4.

X-Wing on 9 across rows 3, 9. Result: remove 9 from R1C1, 9 from R7C1, 9 from R1C8, 9 from R7C8.

Play this puzzle and look for the move yourself. It's a practice game, so it doesn't count towards your stats.

How to spot it

1. Pick a number and look at each row: where can it go?
2. Find two rows where it has exactly two possible cells, in the same two columns.
3. Remove the number from all other cells in those two columns.
4. Try the same with columns: two columns where the number fits in the same two rows.

Tips

• The name comes from the X you get by joining the opposite corners of the rectangle.
• Look for X-Wings on numbers that are already placed many times: fewer candidates means a cleaner pattern.

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