How to solve an XY-chain in Sudoku
An XY-chain is a sequence of two-candidate cells, each sharing a digit with the next, that removes a digit from cells that see both ends.
How it works
Each cell holds two digits. If the first cell is not A, it must be B. That forces the next cell, which shares B, to take its other digit, and so on down the chain.
The first cell is A, or the last cell is A. Any cell that sees both ends cannot be A. An XY-Wing is the shortest XY-chain.
How to spot it
Pick a two-candidate cell and follow the cells that share a digit with it, looking for a chain that comes back to the same digit.
A worked example
This is a step from a real puzzle, a moment where the next move needs an XY-chain.
A and B are the two ends of the chain; W are the cells linking them. Struck-through digits are the ones that can be removed.
R2C1 (6/9) → R5C1 (6/7) → R5C5 (1/7) → R9C5 (1/2) → R8C6 (2/6) → R8C3 (6/9) is a chain where each cell shares a digit with the next. If R2C1 isn't 9, it must be 6. That forces R5C1 to be 7, then R5C5 to be 1, then R9C5 to be 2, then R8C6 to be 6, and finally R8C3 to be 9. So either R2C1 or R8C3 is 9. Remove 9 from cells that see both: R2C3.
Where it fits in Rocksled Sudoku
Puzzles that need an XY-chain are never rated below Expert. Each step of this kind adds 100 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.