sudokUI › Sudoku techniques › ALS-XZ
ALS-XZ in sudoku
ALS chain of two sets · VWXYZ-Wing is a special case · how it fits
Two almost locked sets, each within one unit and sharing no cell, both contain two digits, X and Z, and every X in one sees every X in the other.
Why it works
X cannot be placed in both sets, so at least one set is left with as many candidates as cells and must contain Z. So Z is removed from every cell outside both sets that sees every Z in both sets.
How to spot it
Start with small sets such as bivalue cells, find two that share two digits, then test whether one digit's cells all see each other.
Worked example
ALS-XZ: sets r8c2, r9c1 and r7c8, r8c9 share restricted common 1; digit 5 can be removed from cells seeing every 5 of both sets. So 5 is removed from r7c1.
Practise it on a real puzzle. sudokUI generates a puzzle that needs ALS-XZ and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs ALS-XZ, the puzzle's difficulty rating grows by 300. Its class is Unfair. It is needed in 1 in 37 puzzles that sudokUI generates. How the rating works.