XY-Wing

The shape

You need three cells with exactly two candidates each. Call the pivot XY. One wing shares a unit with the pivot and holds XZ. The other wing also shares a unit with the pivot and holds YZ. The two wings do not need to see each other.

The conclusion: any cell that both wings can see — that shares a unit with each of them — cannot contain Z.

Why Z has to go

The pivot is either X or Y, with no third option. If it is X, the first wing cannot be X, so it is Z. If it is Y, the second wing cannot be Y, so it is Z. Either way one of the wings ends up as Z.

A cell seeing both wings therefore shares a unit with a Z, no matter how the pivot resolves. That cell cannot be Z. Notice you never learn which wing is Z, and you never need to.

How to find one

Start from cells with exactly two candidates — on a stuck Expert or Master grid there are rarely more than eight or ten. Treat each as a pivot and look at the two-candidate cells it can see. You need one sharing X, one sharing Y, and both carrying the same third digit Z.

Then check the intersection: the cells both wings see. If any of them still lists Z, you have an elimination. If none does, the pattern is real but useless — move to the next pivot.

Relation to other techniques

The XY-Wing is the smallest chain technique on the site. An X-Wing is about one digit in a rigid geometric shape; the XY-Wing is about three digits and uses logical links instead of alignment. It is the natural stepping stone before alternating inference chains.