X-chain
MasterAn X-chain follows one digit from cell to cell. Its links alternate: a strong link, where a unit has only two places for the digit (if one isn’t it, the other is), then a weak one, where two cells just see each other. A chain that starts and ends with a strong link has the digit at one end or the other.
How to spot it
- Pick one digit and find its strong links: units where it has exactly two places.
- Join them into a chain, alternating strong links and cells that see each other.
- With strong links at both ends, remove the digit from cells that see both ends.
Why it works
If the first cell isn’t the digit, its strong link makes the next one the digit, which rules out the cell after it, and so on to the last cell, which is then the digit. Either the first end or the last holds it.
Example
5
4
9
8
1
7
2
7
5
1
4
1
8
7
4
6
5
9
2
3
7
9
7
1
6
4
5
7
7
1
8
5
3
6
4
3
7
3
1
7
8
Follow a chain of 3s.
Common mistakes
- Ending the chain on a weak link: then nothing is proved about the last cell.
- Using two weak links in a row.