How to solve an X-chain in Sudoku
An X-chain links two-place spots for one digit end to end, and removes the digit from cells that see both ends.
How it works
Follow a chain of alternating links for a single digit: in each strong link, one end must be the digit, and between links, cells that see each other cannot both be it.
Each end of a chain of this kind is true or the other end is. So any cell that sees both ends cannot hold the digit. Skyscrapers, two-string kites and turbot fish are short X-chains.
How to spot it
Start with a digit that has several two-place units and try to link them end to end.
A worked example
This is a step from a real puzzle, a moment where the next move needs an X-chain.
A and B mark every other cell along the chain. The two ends are one of each. Struck-through digits are the ones that can be removed.
The 5s link up along R7C3, R7C8, R2C8, R2C7, R5C7, R5C5, R6C6, R9C6. Each solid link (two cells that are the only places for 5 in a row, column or box) holds exactly one 5, and cells that see each other can't both hold it. If R7C3 isn't 5, then R7C8 must be 5, which rules out R2C8, so R2C7 must be 5, which rules out R5C7, so R5C5 must be 5, which rules out R6C6, so R9C6 must be 5. So one of the two ends, R7C3 or R9C6, must be 5. Remove 5 from cells that see both: R9C1.
Where it fits in Rocksled Sudoku
Puzzles that need an X-chain are never rated below Expert. Each step of this kind adds 90 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.