Expert

XY-Chain

What it is

A chain of two-candidate cells – whichever way it goes, one end holds the same digit.

How to spot it

Start in a two-candidate cell, step to a cell sharing one of its digits, and continue. If the first and last cell both hold the same digit X, and the chain forces one of them to be X, cells that see both lose X.

Example

Basis
Removed / placed

r1c1 (1 2) → r1c6 (2 3) → r5c6 (3 4) → r5c2 (4 1). If r1c1 isn't 1, it's 2, then r1c6 is 3, r5c6 is 4 and r5c2 is 1. So r1c1 or r5c2 is 1, and 1 is removed from r1c2 and r2c2.

Used from level: Expert