XY-chain
MasterAn XY-chain links cells that have exactly two candidates, each seeing the next and sharing one digit with it. If the first cell isn’t Z, it is its other digit, which forces the next cell, and so on along the chain. If the last cell then has to be Z, one end of the chain is Z for sure.
How to spot it
- Start from a cell with two candidates, one of them Z.
- Move to a cell it sees that shares its other digit, and keep going, each cell forcing the next.
- When a cell’s remaining digit is Z, remove Z from cells that see both ends.
Why it works
Each cell has only two candidates, so if it isn’t one, it’s the other. Following that from the first cell, either the first cell is Z or the last one is.
Example
3
4
8
2
1
2
6
4
5
9
2
7
4
3
1
5
6
3
8
7
9
3
6
7
9
5
4
6
8
3
4
3
8
2
7
2
4
6
8
3
1
7
2
4
Follow a chain of cells with two candidates each.
Common mistakes
- Using a cell with three candidates: the chain only works with exactly two.
- Linking two cells that don’t see each other.