X-Wing
A digit's homes in two rows form a 2×2 box across the same two columns.
X-Wing is the smallest of the "fish" patterns. Pick a digit. Find two rows where the digit's remaining candidate cells fall in exactly the same two columns. Whichever way the digit ends up arranged, those two columns are guaranteed to receive it (once each, one per row) — so the digit can be erased from those columns in every other row.
The pattern works just as well swapped: two columns that confine a digit to the same two rows force eliminations across those rows.
X-Wing is the first move that asks you to think in two dimensions at once. After it clicks, the rest of the fish family makes sense.
When the move applies
When pairs and locked candidates have run their course, switch to a digit-by-row sweep. Two rows whose candidate column-lists match exactly is the tell.
The procedure
- Pick a digit.
- For each row, list the columns where that digit is still a candidate.
- Find two rows whose lists are identical and contain exactly two columns.
- Remove the digit from those two columns in every other row.