An X-Wing is the first big single-digit pattern — you only watch one digit’s positions.
In two different rows, that digit can only appear in the same two columns (or two columns with the same two rows). The four corners form a rectangle — like the letter X if you draw both diagonals.
Why it works
In each of the two rows, the digit has only two possible seats, and those seats share columns. So in each row, the digit will take one of those two columns.
There are two possible worlds:
• one diagonal is true, or • the other diagonal is true.
The two rows cannot both pick the same column. So either way, both special columns get the digit. Therefore no other cell in those two columns can also hold it.
(You can also prove it by contradiction: assume a victim cell is the digit → one fish row loses all homes.)
How to spot it
Restriction in rows → eliminate in columns (and vice versa). Notate where a digit is bi-local first.
Demonstration
Pick one digit. Look for two rows where it only appears in candidates in the same two columns.
The green cells lock 5 to this rectangle. In each row, the digit’s homes shrink to those two columns.
World A: one diagonal. World B: the other diagonal. Either way, both columns already get 5 — the rows cannot share a column.
So those two columns are spoken for. That rectangle pattern is called an X-Wing.
Eliminate 5 from the rest of those columns (red). Outside the rectangle, those columns are already occupied in both worlds.
Example 1
For one digit, find two lines that share the same two crossing lines.
This rectangle is an X-Wing on 1.
Remove 1 from the rest of those lines (red). Why? Both diagonals leave both fish lines occupied.
Example 2
For one digit, find two lines that share the same two crossing lines.
This rectangle is an X-Wing on 4.
Remove 4 from the rest of those lines (red). Why? Both diagonals leave both fish lines occupied.