Sudoku technique · First needed at: Extreme ●●●●●●●●

XY-chain

A run of two-candidate cells that starts and ends on the same number: one end must be it, so anything seeing both ends is out.

An example from a real puzzle

How to spot it

1. Start at a cell with two candidates and follow a chain of two-candidate cells, each sharing a number with the next.
2. If the chain starts and ends on the same number X, any cell that sees both ends cannot be X.

Why it works

If the first cell is not X, each link forces the next, until the last cell is X. So one end is always X.