Swordfish
Three rows where a digit can only fit within the same three columns. Eliminate that digit from the rest of those columns. The pattern is symmetric — three columns × three rows works the same way.
Advanced sudoku technique
What it is
Swordfish is X-Wing's older sibling: same logic, one row and one column deeper. Pick a digit. Find three rows in which that digit's only possible cells lie within the SAME three columns (a row can have the digit in only two of the three columns — that's still in-pattern, as long as it stays inside the column set). Each of those rows must place the digit somewhere in those three columns, and the digit appears at most once per column. The three placements therefore distribute one per row × one per column across the 3×3 grid of corners. So the digit cannot appear in those three columns in any OTHER row. The column-rooted variant is symmetric: three columns confining a digit to three rows prune the digit from the rest of those rows.
When to use it
On expert and master puzzles when X-Wing is exhausted but eliminations are still needed. Maintain accurate candidates and scan one digit at a time for the 3×3 row/column lock.
Worked example
Digit 4's candidate cells in rows 2, 5, and 8 all lie within columns 1, 4, and 7. Row 2 has 4 in columns 1 and 4. Row 5 has 4 in columns 1 and 7. Row 8 has 4 in columns 4 and 7. The three placements of 4 must distribute one per row and one per column across that 3×3 grid. Therefore 4 can be eliminated from columns 1, 4, and 7 in every other row (rows 1, 3, 4, 6, 7, 9).
Try it
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