Difficulty
Hard
Sudoku technique guide
Swordfish
Sometimes the obvious moves disappear, but the grid is still giving you a useful clue. Swordfish is a calm way to follow it.
By the end, you will know what to notice, how the idea works and when it is worth using.
At a glance
Technique family
Fish
Purpose
Removes candidates
Typical puzzle stage
Late
Frequency
Uncommon
Prerequisites
X-Wing and candidate maps
Why this technique matters
Swordfish is the next step after X-Wing. It shows how a candidate can be restricted across three lines, not just two.
The idea in plain English
A Swordfish occurs when a digit in three rows is confined to the same three columns, with two or three candidate positions in each row. That digit can be removed from other cells in those columns.
How to recognise it
Look for a three-by-three line pattern. It will usually be uneven: one row may have two candidate positions and another three.
Pause when you see this shape. You do not have to force it; just check whether the pattern is complete.
How the logic unfolds
Think of this as a short conversation with the grid. Take one check at a time, and let each answer tell you what to inspect next.
Start by. Choose one candidate digit.
Then. Find three rows where its candidates fall only within the same three columns.
Next. Check that no selected row has a candidate outside those three columns.
Finally. Remove the digit from other cells in those columns.
When it usually appears
Try it after X-Wing and simpler chains are not available, especially when one digit has a distinctive scattered pattern.
Common traps
These are easy places to get turned around. They are useful checks, not signs that you are doing badly.
- Demanding exactly three candidates in every row. Swordfish lines may have two or three.
- Eliminating from the selected rows rather than from the shared columns outside the pattern.
Frequently asked questions
Can Swordfish be column-based?
Yes. Three columns can restrict a digit to three rows.
Is every three-by-three pattern a Swordfish?
No. The chosen lines must contain all their candidates within the three shared cross-lines.
Quick takeaway
- A Swordfish belongs to the fish family.
- Its purpose is to remove candidates.
- Look for a complete pattern, not one that only nearly fits.
- It is most useful late.
- Once you make the deduction, return to the simpler scans.