A Y-Wing is a tiny story with three cells — also called XY-Wing, and sometimes a bent triple (three numbers in three cells that are not in a straight line).
A pivot cell has candidates XY. It sees two wings: one is XZ, the other is YZ. The letter Z is the shared sting the pivot does *not* have.
Why it works
The pivot is either X or Y.
• If the pivot is X, then the XZ wing cannot be X — so that wing must be Z. • If the pivot is Y, then the YZ wing must be Z.
So no matter what, one of the wings becomes Z. Any cell that can see both wings would see a Z either way — it cannot be Z itself.
Contradiction view: assume a victim is Z → both wings lose Z → pivot is forced into impossibility. Same idea as a short AIC that starts and ends on Z.
How to spot it
Only scan bi-value cells. Look for a bent triple of candidates; find shared letters first, then hunt the third cell in intersections. Can span four boxes when the pivot is not boxed with a wing.
Demonstration
Start from the rules: each of these bi-value cells must be one of its two digits. Find a pivot XY that sees two wings.
One wing is XZ, the other is YZ — a bent triple. The sting digit Z is 1.
World A: pivot is X → the XZ wing must be 1. World B: pivot is Y → the YZ wing must be 1.
Both stories force a wing to be 1. That pattern is a Y-Wing (XY-Wing).
Eliminate 1 from cells that see both wings (red). They get stung in every possible story. (If a victim were 1, both wings would break and the pivot would have nowhere to go.)
Example 1
Find a bi-value pivot with two bi-value wings (bent triple).
This Y-Wing stings with 8.
Remove 8 from cells that see both wings (red). Both pivot cases force a wing to be that digit.
Example 2
Find a bi-value pivot with two bi-value wings (bent triple).
This Y-Wing stings with 1.
Remove 1 from cells that see both wings (red). Both pivot cases force a wing to be that digit.