X-Wing

X-Wing is the first advanced technique most players learn. It looks at one digit across two rows at once, and it is easier to spot than its name suggests.

What is an X-Wing?

Pick a digit. Suppose that in two different rows, it can only go in the same two columns. The four cells form a rectangle. In the first row, the digit is in one of the two columns; in the second row, it must be in the other one (the same column cannot hold it twice). Either way, both columns get their copy of the digit from these two rows. So no other cell in those two columns can hold it.

Example

An X-Wing on 7 in rows 2 and 6 clears columns 3 and 7.

Look at digit 7. Row 2 has five empty cells, but columns 1, 5 and 9 already have a 7 lower down (R7C1, R8C5, R9C9). So in row 2, 7 can only go in R2C3 or R2C7. Row 6 is the same: only R6C3 or R6C7.

If row 2's 7 is in column 3, row 6's 7 must be in column 7, and the other way round. Both ways, columns 3 and 7 each get their 7 from rows 2 and 6. Erase 7 from every other cell in those columns: R1C3, R3C3, R4C3, R5C3 and the same rows in column 7.

How to find an X-Wing

  1. Pick a digit that appears often on the board.
  2. Find rows where that digit has exactly two possible cells.
  3. Look for two such rows whose cells are in the same two columns.
  4. Erase the digit from the rest of those two columns.

It works the other way too: two columns where the digit fits in the same two rows let you clear those rows.

Tip: only try X-Wing once singles and pairs have run out. On expert puzzles, check each digit in turn; the pattern usually stands out once you write the candidates for that one digit.

Where it fits

X-Wing builds on pointing pairs and is the gateway to bigger patterns such as Swordfish (three rows and three columns). Our Sudoku app's hints point out X-Wings when they are the next move on expert puzzles.

Practise on your phone

Our free Sudoku app has hints that point out the technique you can use next, with the cells highlighted. It plays offline.

Get it on Google Play