Swordfish

Difficulty level: expert

A Swordfish forms when three rows each restrict a number to cells drawn from the same three columns. The number must appear exactly once in each of those rows and columns, weaving a three-strand lattice.

However the placements resolve, the three columns each receive the number from one of the three rows — so every other cell of those columns can drop the candidate.

Like the X-Wing, the pattern transposes: three columns restricting a number to three shared rows eliminates along the rows. Spotting Swordfish reliably is a hallmark of expert solvers.

Worked example

If 3 fits only in the highlighted cells of rows 1, 4 and 9 — all within columns 2, 5 and 8 — the 3s are locked to that lattice, clearing 3 from the rest of those columns.
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How to apply it

  1. Pick a number and find rows where it has two or three candidate cells.
  2. Look for three rows whose candidates fit within the same three columns.
  3. Erase the number from all other cells of those three columns.
  4. Check the affected columns for forced placements.

Practice it

The fastest way to internalize the swordfish is to use it. Play a free expert puzzle — the in-game hint system points out exactly this pattern when it appears, or browse the full technique library.