Swordfish in Sudoku: The Natural Step After X-Wing

A Swordfish uses three rows and three columns for one digit. Across those rows, the candidate appears in no more than three columns in total.

The pattern is powerful because each base row must place the digit somewhere within the same three columns.

Choose one digit

Start with a digit whose candidate positions are limited in several rows. Record the columns used by each row rather than trying to see the whole grid at once.

Build three base rows

Find three rows whose combined candidate columns contain exactly three columns, with each row holding two or three positions. The equivalent column-based pattern works the same way.

Reason across the fish

Each base row needs one occurrence of the digit, and all its possibilities lie in the three cover columns. Those columns therefore contain the digit’s placements for the fish rows, allowing eliminations elsewhere.

Eliminate outside the base

Remove the digit from other rows in the three cover columns. Leave all candidates in the three base rows untouched, even when they look redundant.

Use it sparingly

Swordfish comparisons are expensive. Check singles, locked candidates and pairs first, then scan one digit with a clear candidate map. A failed scan is a cue to return to easier work.