XY-Wing

This is where three squares have the same three numbers (each square containing exactly two of the numbers) as possibilities and are linked by either being in the same block, row or column. Consider the following board:

In row seven, column four, the two possibilities are (5, 6). If that square is a 5, that forces the square in row nine, column five to be a 7. Similarly if the square at row seven, column four is a 6, that forces the square at row seven, column eight to be a 7. This means that in block eight, we can eliminate 7 as possibility from the squares that are in row seven. This is because that no matter which of our two possible squares is a 7, the squares in block eight, row seven cannot be a 7. By this same reasoning we can eliminate 7 as a possibility from the squares that are in block nine, row nine.

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