Proven Sudoku
Techniques / Almost locked sets
Aligned Pair Exclusion
Also called APE.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Look at two squares together and list every pair of digits they could hold. Cross out the pairs blocked by nearby squares. If a digit never survives in any remaining pair, it can go.
The exact rule, for stronger players
The rule, stated exactly: Two cells that see each other, plus the ALSs that both see. Any candidate combination for the two cells that would leave some seen ALS with no valid arrangement is impossible, and combinations impossible for every arrangement eliminate candidates.
What it removes: Remove candidates that appear in no surviving combination.
Why it works: Enumerate the candidate pairs for the two cells. Discard any pair that the surrounding sets cannot accommodate. Any digit absent from every surviving pair for a cell is eliminated from that cell.
Worked examples
None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.
Learn these first
- Almost Locked SetA small group of squares holding exactly one number more than there are squares.
Related almost-locked sets
- Aligned Triple ExclusionLook at three squares together and list every set of three numbers they could hold.
- Almost Hidden SetThe mirror image of an almost locked set.
- Almost Locked CandidatesTwo areas cross.
- ALS ChainA row of almost full groups linked one to the next.
- ALS-XY-WingThree almost full groups arranged like a wing.
How often does it come up?
Our rough estimate: uncommon, and tedious by hand because it requires enumerating combinations.