Extreme Used in extreme puzzles.
X-Chain
Strong and weak links, in turn
Pick a digit. A strong link joins its only two places in a unit: if one is not the digit, the other is. A weak link joins two cells that see each other: if one is the digit, the other is not.
An X-Chain goes from cell to cell by links strong and weak in turn, starting and ending with a strong one. If its first cell is not the digit, the second is, so the third is not, the fourth is, and so on to the last, which is. So the digit is in the first cell or in the last.
A cell that sees both ends cannot hold the digit: remove it there. The shortest X-Chains are four cells long: the Skyscraper, which has a chapter of its own, the 2-String Kite and the Turbot Fish.
What to look for
Pick a digit, start from a unit where it has two places, and follow strong and weak links in turn to another such unit.
Example
- The cells the technique relies on
- Cells where it removes candidates, struck out in red
- Strong link (solid line) and weak link (dashed)
Practise it
In the app, Learn › X-Chain sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.