Aligned Pair Exclusion
Two cells that don't share a unit are checked against every "almost locked" set nearby — any combination of their candidates that would leave one of those sets with no valid completion is ruled out.
The solver does not attempt this technique, so Smart Hint will never suggest it. It is documented because you may still want to spot it yourself.
How it works
- 1A1 and E2 share no row, column or box, so nothing stops them holding the same digit — but each of them sees both E1 and F1.
- 2E1 and F1 hold {3, 5, 7} between two cells: an Almost Locked Set, one digit short of locking.
- 3Try A1 = 5 with E2 = 3. E1 loses 5 to A1 and 3 to E2, leaving 7; F1 loses 5 to A1, also leaving 7 — two 7s in row 1.
- 4Try A1 = 5 with E2 = 7 instead, and F1 loses both its candidates. Every pairing that starts A1 = 5 breaks, so 5 goes from A1.
- Cells that establish the pattern
- Where the conclusion lands
- Candidate ruled out