The idea
You need three cells, each with exactly two candidates:
- A pivot holding {A,B}.
- A pincer the pivot can see, holding {A,C}.
- Another pincer the pivot can see, holding {B,C}.
The pivot is either A or B. If it is A, the first pincer cannot be A, so it is C. If it is B, the second pincer cannot be B, so it is C. Either way, one of the two pincers is C — so no cell that both pincers can see can be C.
Whichever digit the pivot takes, one of the two pincers is forced to the shared digit — so it is removed from the marked cell, which both pincers can see.
Reading the shape
The three cells do not have to be near each other. The pivot must share a unit with each pincer, but the pincers need not share one with each other. Typically the pivot sits in a box with one pincer and in a row or column with the other.
Finding one
- Mark all candidates and note every cell with exactly two.
- Take one as a pivot and list the two-candidate cells it can see that share exactly one digit with it.
- Look for two of those whose remaining digits match — that shared digit is the one you eliminate.
- Find the cells both pincers can see. Any of them holding the shared digit loses it.
Why it is worth learning before Swordfish
Two-candidate cells stand out on a marked-up grid, so in practice an XY-Wing is often easier to find than a Swordfish, even though it looks more complicated written down. On expert puzzles it is frequently the move the grid is waiting for.
Common questions
What is an XY-Wing?
A pivot cell with candidates {A,B} and two cells it can see holding {A,C} and {B,C}. The digit C can be removed from every cell both pincers can see.
Do the three cells need to be in the same box?
No. The pivot must share a unit with each pincer; the pincers themselves can be anywhere.
Is an XY-Wing the same as a Y-Wing?
Yes — the two names describe the same pattern.
Is it harder than an X-Wing?
It is harder to describe but often easier to spot, because two-candidate cells are conspicuous on a fully marked grid.