Sudoku technique guide

XY-Chain Sudoku Technique

An XY-Chain links cells that each have exactly two candidates. This guide explains the idea in plain English, when to look for it, and the common mistake to avoid.

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At a glance

Difficulty

Advanced

This is where XY-Chain normally sits in a human solving path.

Family

Chain techniques

It belongs to the chain techniques group of Sudoku logic.

Main action

Usually remove candidates

Most technique moves reduce notes first. That often reveals a simpler placement afterwards.

What is XY-Chain?

An XY-Chain links cells that each have exactly two candidates.

The important thing is that this is logic, not guessing. You are using the current candidates to prove that a number must either go in one place, or cannot stay in another place.

In SudoSketch, this technique is designed to work with notes and Coach highlighting, so the key cells and removal cells can be shown clearly.

How XY-Chain works

Find cells with exactly two candidates.
Link them so each step shares a candidate with the next cell.
If the chain starts and ends with the same candidate, any cell seeing both ends can lose that candidate.

When to look for it

Look for XY-Chain after easier moves have stopped working. First check for naked singles, hidden singles, locked candidates and simple pairs. If those do not move the puzzle forward, this technique may be worth checking.

For learning, do not try to scan for every advanced technique at once. Pick one method, understand the shape, and practise spotting that one pattern.

Common mistake

Every cell in the chain must have exactly two candidates.

If you are unsure, rebuild the candidates first. Bad notes create bad logic, and Sudoku will absolutely punish you for it like a tiny spreadsheet goblin.

XY-Chain FAQ

What is XY-Chain in Sudoku?