Swordfish and Jellyfish

Master

Swordfish is an X-Wing with three rows: a digit whose places in three rows all lie in the same three columns. Jellyfish does it with four. Both are Master techniques.

Swordfish

Find three rows where a digit has two or three places, all within the same three columns. The three rows need three copies of the digit, no two in the same column, so they take one cell in each of the three columns. Those columns are then complete for that digit, and it goes from their other cells.

Not every row needs all three columns: places in columns 1 and 4, 4 and 8, and 1 and 8 still make a Swordfish. That’s what makes it harder to see than an X-Wing.

999999999999999999964589253883561424516638145241867265488463972511A2B3C4D5E6F7G8H9I
Only the notes for 9 are shown. In columns 2, 4 and 8, a 9 fits only in rows D, E and H. Those three 9s can’t share a row, so each of rows D, E and H gets one of them from here, and the 9s in D7 and H1 go.

Jellyfish

Four rows whose places for a digit lie in four columns. They’re rare: when one turns up, the same reasoning clears the digit from the rest of the four columns.

3333333333333333333333335487245282819574821452869582824349852219481A2B3C4D5E6F7G8H9I
Only the notes for 3 are shown. In rows C, D, F and H, a 3 fits only in columns 2, 4, 5 and 9. Those four 3s can’t share a column, so each of columns 2, 4, 5 and 9 gets one of them from here, and the 3s in E4 and E9 go.

How to spot them

  • Show one digit’s notes, then list the rows with two or three of them. Three rows that between them use only three columns are a Swordfish.
  • Try columns too: the pattern works in both directions, and one is often easier to see.

Try it on a real puzzle

Master puzzles in Sudoku Knockout need techniques like this one, and never a guess. When you’re stuck, a hint shows you the step.

Play a Master puzzle