Sudoku techniques · Master
XY-Chain in Sudoku
An XY-Chain is a run of bivalue cells where each cell shares one candidate with the next. Follow the chain and the links force at least one of its two ends to hold the candidate the ends have in common, so any cell that sees both ends can drop it.
Why it works
In an XY-chain, the links force at least one of its two ends to hold their shared candidate, so any cell that sees both ends loses it.
How to spot it
A Y-Wing is an XY-Chain three cells long. Longer chains work the same way and reach further.
- Collect every bivalue cell; those are your only building blocks.
- Step from cell to cell through a shared candidate, and make sure consecutive cells actually see each other.
- Stop when the first and last cell share a candidate. That one is what you eliminate.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
A chain of 4 bivalue cells has candidate 3 in both end cells. If either end is not 3, the links force the other end to be 3.
- 2
At least one end must be 3, so we can remove the candidate 3 from any other cell that sees both ends.