XY-Wing
Three cells with two candidates each force a digit into one of two cells.
A technique for Hard puzzles. Play a Hard puzzle
What it does
Three cells with two candidates each work together. The pivot holds X or Y. One wing, which sees the pivot, holds X or Z; the other wing, which also sees the pivot, holds Y or Z. If the pivot is X, the first wing must be Z; if it is Y, the second wing must be Z. Either way, one of the wings is Z.
How to spot it
Look for a cell with two candidates, then for two cells it sees, each sharing one of its digits and both sharing a third digit between them. The pivot is the cell in the middle; the wings are at the ends.
What it lets you do
Any cell that sees both wings can’t be Z, so you remove Z from it.
Try it
Find the XY-Wing on this board. Pick the cells it clears, mark the candidates it removes, then check.
These are the notes after the easier steps: some candidates are already removed, as they would be in your game.
Practise it
The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Hard level; any Hard puzzle may need it too.