Hard

W-Wing

What it is

Two identical two-candidate cells joined by a strong link remove their other digit from cells that see both.

How to spot it

Find two cells with the same two candidates XY that don't see each other. If X has only two places in some unit, and each of those places sees one of the cells, then one of the cells must be Y.

Example

Basis
Removed / placed

r1c1 and r6c5 both hold 4 and 5. In row 9, 4 only fits in r9c1 and r9c5, which each see one of them. One of those is 4, so r1c1 or r6c5 is not 4 – it's 5. So 5 is removed from r1c5 and r6c1, which see both.

Used from level: Hard