Sudoku solving technique · Expert
XY-Chain
A chain of two-candidate cells whose ends share a digit.
What is an XY-Chain?
A chain of bi-value cells (each with exactly two candidates). Adjacent cells in the chain share one candidate. The two endpoints share a common candidate. Any cell that sees both endpoints can have that candidate eliminated.
How to spot an XY-Chain
Start from a cell with two candidates and step to a cell it sees that shares one of them, then carry on through the other digit each time. If the chain ends in a cell that carries the digit you did not use at the start, one of the two ends holds that digit. It can be removed from cells that see both ends. A Y-Wing is the shortest such chain.
- Difficulty
- In Sudoku Pro, this technique belongs to the Expert level.
- Learning path
- Technique 20 of 20 on the learning path
An example from the app
| Pattern (purple). Candidates 1, 2 | Given 4 | Empty | Empty |
| Empty | Pattern (purple). Candidates 2, 3 | Given 1 | Pattern (purple). Candidates 3, 4 |
| Affected cell (orange). Candidates 2, 4. Eliminated 1 | Empty | Given 3 | Pattern (purple). Candidates 1, 4 |
| Given 3 | Empty | Empty | Given 2 |
Small digits are candidates. Crossed-out digits can be removed in this step.
- Pattern (purple)
- Affected cell (orange)