LeafyOrb

Sudoku techniques · Fiendish level

Swordfish: an X-Wing across three lines

A Swordfish uses one digit whose candidates in three rows all fall within the same three columns. Those three rows must place their three copies of the digit in those columns, so the digit can be removed from the rest of the three columns. It works with rows and columns swapped, too.

How it works

Each of the three rows needs the digit once, and each row can only put it in one of the three columns. Three rows, three columns, one digit each: the columns are filled by these rows, so no other cell in them can take the digit.

Each base row needs two or three candidates for the digit. Not all nine crossings need a candidate, which is what makes a Swordfish hard to see. Ninebloom's hints teach it on Fiendish puzzles.

  • The row, column or box the move uses
  • The cells that make the pattern
  • Candidates the move removes
Swordfish example on a Sudoku board: 4 is confined to the same three lines across row 1, row 5 and row 8. Those three lines must use it there, so 4 comes out of the rest of them.

Example 1 · from Fiendish puzzle #194

The 4 can go in row 1 only in R1C4 and R1C9; in row 5 only in R5C4 and R5C7; in row 8 only in R8C7 and R8C9. All of those cells fall in columns 4, 7 and 9. Three rows need three 4s, and they must take them from these three columns, which only have room for three. Remove 4 from R4C9, R6C9, R2C7 and R6C7.

Play this position in Ninebloom: Home › Your own puzzle, then paste the code. NG-245IKU-UYFTRQ-7A2V6E-B5WMZO-ZO8GUJ-0AVO0N-2WALF2-OSPRO6-JEATT

Cells are named by row and column: R4C3 is row 4, column 3. Boxes are numbered 1 to 9, left to right and top to bottom.

How to spot it

  1. Make sure your pencil marks are complete for the digit you are checking.
  2. List the rows where the digit has two or three candidates, and note their columns.
  3. Find three of those rows whose columns, all together, are only three columns.
  4. Remove the digit from every other cell in those three columns.

Common mistakes

  • A base row with four or more candidates. Every base row must stay inside the three columns.
  • Expecting a full 3 by 3 grid of candidates. Two per row is enough, as long as all of them are in the three columns.
  • Looking for it too early. Singles, pairs and X-Wings are far more common, so try those first.

Questions

Is there a bigger fish than a Swordfish?

Yes, a Jellyfish uses four rows and four columns. It works the same way but is rare in practice.

Why is the Swordfish so hard to see?

Its candidates do not form a neat shape. Some crossings are empty, so it is easiest to find by writing down, for one digit, which columns each row allows.