ALS-XY-Wing
Three Almost Locked Sets in a row: two outer sets hang off a middle "bridge" set, and a digit shared by the outer two must land in one of them.
How it works
- 1Three Almost Locked Sets lie along row 1, each one digit short of locking: A1 + B1 + C1 on {2, 3, 5, 6}, the bridge E1 + F1 on {6, 7, 8}, and G1 + H1 on {2, 8, 9}.
- 2The first set and the bridge share 6, and each holds it in a single cell — A1 and E1, both in row 1 — so only one of the two can use it. The bridge and the last set share 8 the same way, at F1 and G1.
- 3Suppose 2 were false in both outer sets. Each would then have as many digits as cells and lock: the first onto its 6, the last onto its 8. That robs the bridge of 6 and of 8, leaving two cells only the 7 — impossible.
- 4So 2 lands in B1/C1 or at H1. D1 shares row 1 with all of them, so it can't be 2.
- First colour in the chain
- Second colour in the chain
- Cells that establish the pattern
- Where the conclusion lands
- Candidate ruled out