sudokUI › Sudoku techniques › Aligned Pair Exclusion
Aligned Pair Exclusion in sudoku
XYZ-Wing is a special case · how it fits
Two cells that see each other, together with one or more almost locked sets, bivalue cells included, in which every cell sees both of the two cells.
Why it works
The two cells cannot hold the same digit, nor two digits that are both candidates of one such ALS, which would leave it a digit short. A candidate of either cell is eliminated when all its pairings with the other cell's candidates are ruled out.
How to spot it
Pick two cells in the same unit with few candidates, then look for bivalue cells or small ALSs seen by both.
Worked example
Aligned Pair Exclusion: every candidate pairing of r7c4 and r8c4 that survives the sets they both see leaves no place for the removed candidate.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Aligned Pair Exclusion 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 Aligned Pair Exclusion, the puzzle's difficulty rating grows by 320. Its class is Unfair. It is needed in 1 in 16,000 puzzles that sudokUI generates. How the rating works.
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