How to solve an ALS-XZ in Sudoku
An ALS-XZ links two almost-locked sets with a digit they cannot both hold, which forces a second digit into one of them and clears it from cells that see both.
How it works
An almost-locked set (ALS) is a group of cells in one row, column or box that hold one more digit between them than they have cells, such as a single cell with two notes, or three cells with four digits between them. If you take one of those digits away, the group is locked: each cell gets a digit of its own.
Take two such sets that share a digit X, where every X in one set can see every X in the other, so they cannot both hold X. Whichever set does not have X is locked on its other digits. If the two sets share a second digit Z, that locked set holds Z. So one of the two sets has Z, and any cell that sees every Z in both can drop it.
How to spot it
Start with a two-note cell and look for a group of cells somewhere that holds those two digits and one more. This one is hard to spot by eye; it is a technique that pencil notes make easier.
A worked example
This is a step from a real puzzle, a moment where the next move needs an ALS-XZ.
A are the cells of one almost-locked set, B the other. Struck-through digits are the ones that can be removed.
R2C3, R2C6 hold 3 digits in 2 cells, and so do R5C3, R5C4. The 5s in the two groups all see each other, so they can't both be 5. Whichever group has no 5 has just enough digits left to fill its cells, and 9 is one of them. Either way one of R2C6, R5C3, R5C4 is 9, so remove 9 from cells that see all of them: R5C6.
Where it fits in Rocksled Sudoku
Puzzles that need an ALS-XZ are never rated below Expert. Each step of this kind adds 104 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.