X-Wing

X-Wing is the first 'fish' pattern. Pick a digit. Find two rows where it can only go in the same two columns. Whatever ends up where, those two columns are guaranteed to contain that digit — once in each row. So the digit can be erased from those columns everywhere else.

The pattern works symmetrically for columns: two columns confining the digit to the same two rows imply elimination across those rows.

X-Wings are the entry point to advanced sudoku. Once you spot them you'll see them often on hard puzzles.

When to look for it

When pairs and locked candidates run out, scan each digit row by row, listing the columns it can still occupy. Two rows with the same pair of columns is an X-Wing.

How to apply it

1. Pick a digit.
2. For each row, list the columns that can still hold it.
3. Find two rows where that list has exactly two columns, and the columns match.
4. Eliminate the digit from those two columns in every other row.

Example