X-Wing

If a number can go in only two cells in each of two rows, and those cells sit in the same two columns, the number must land on opposite corners of that rectangle. Whichever diagonal it takes, both columns get their copy, so no other cell in those columns can be that number.

How to spot it

1. Choose a number and find a row where it has exactly two possible cells.
2. Find a second row where it has exactly two cells in the same two columns.
3. Remove the number from every other cell in those two columns.

A real example

3 has only two possible cells in row 1 and in row 8, both times in columns 4 and 7. It must take opposite corners of that rectangle, so columns 4 and 7 can’t hold 3 anywhere else: r7c7 loses 3.

Tips

• It works the same with rows and columns swapped.
• Look for it when a number has few candidates left across the whole board.

Practice on Expert puzzles