What is XY-Wing? How to Solve Sudoku

What it is

Built from three cells with two candidates each.

• The pivot: 1 or 2

• Cell A (sees the pivot): 1 or 3

• Cell B (sees the pivot): 2 or 3

If the pivot is 1, A is 3. If the pivot is 2, B is 3. So A or B is always 3.

That means any cell that sees both A and B can't be 3.

How to spot it

Start from a two-candidate cell and look for two more two-candidate cells it sees. Three cells, three digits, each pair sharing one digit.

Example

Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.

R2C3 can only be 3 or 6. If it's 3, R3C2 becomes 4. If it's 6, R2C5 becomes 4. So one of R3C2 and R2C5 is always 4, and any cell that sees both (R3C6) can't be 4.