Academy
SwordfishSwordfish sudoku technique
A swordfish extends the X-Wing from two lines to three. Take a digit confined, in each of three rows, to cells that all fall within the same three columns — two or three cells per row both count. Those three columns must then supply the digit to the three rows, which uses up the columns entirely: any other candidate for the digit in those columns can be removed.
How to find a swordfish
- Choose a digit and note which cells can take it in every row.
- Search for three rows whose candidates for the digit fit inside a shared set of three columns.
- Erase the digit from those three columns outside the pattern cells.
- No luck row-wise? Transpose the search and look across columns instead.
The pattern on the grid
A real position — every pencil mark computed, every red candidate a verified elimination. Toggle to see the grid after the move.
Swordfish on 1: r2c7, r2c8, r2c9, r6c7, r6c9, r9c7, r9c9 => r7c7<>1, r7c8<>1, r7c9<>1, r8c7<>1, r8c8<>1, r8c9<>1
534719286896000000127068400482197653315826040769003020200670000600980000908004060
Standard 81-character format (0 = empty) — paste it into your solver of choice and verify the move yourself.
Worked example
Map digit 1 across the grid below and the nine-or-fewer pattern cells r2c7, r2c8, r2c9, r6c7, r6c9, r9c7, r9c9 emerge: each defining line holds the digit only inside the shared cover lines. Wherever the 1s finally land within the pattern, the cover lines are used up — which erases r7c7<>1, r7c8<>1, r7c9<>1, r8c7<>1, r8c8<>1, r8c9<>1.
When it fails
Rows with two candidate cells are the reliable skeleton. A row with four candidates can never be part of the fish, and a 'swordfish' whose columns number four is no swordfish at all — re-count before erasing.