---
title: "Swordfish: Follow one digit across three lines"
description: "A Swordfish confines a digit’s candidates in three starting rows to exactly three columns. The three required placements use those columns."
lang: "en"
canonical: "https://unisu-sudoku.app/en/sudoku-techniques/swordfish/"
---

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Further techniques

# Swordfish: Follow one digit across three lines

A Swordfish confines a digit’s candidates in three starting rows to exactly three columns. The three required placements use those columns.

![Unisu lesson on Swordfish, showing the explained grid](https://unisu-sudoku.app/_astro/en.3CsO7Tn7_Z5j32k.webp)

A Swordfish confines a digit’s candidates in three starting rows to exactly three columns. The three required placements use those columns.

## An example from the app

In row 1, row 4, row 7, candidate 9 is confined to column E, column H, column D. There are 3 starting lines and exactly 3 crossing lines.

The 3 required placements of 9 occupy column E, column H, column D. In those lines, 9 is ruled out outside row 1, row 4, row 7.

These notes are therefore ruled out: E2: 9; E6: 9; D6: 9.

The image shows the candidates relevant to this step. A note being hidden does not mean it has been logically eliminated.

## Practice the next step

There must be no candidates for the digit outside those columns in the starting rows. Each row does not have to contain three candidates.

In the app, follow the reasoning in the matching lesson and try a task yourself. With a new puzzle, always start from its actual candidate state.

## Questions about Swordfish

### Does every starting row need three candidates?

No. Some rows can have fewer positions. Together, all positions of the digit in the three rows must be limited to three cover columns.

### Must Swordfish form a regular rectangle?

No. The three starting and cover areas matter, not a completely filled geometric shape.

### Where do row-based Swordfish eliminations occur?

In the three cover columns outside the three starting rows. Those positions cannot hold the digit.

### Can Swordfish start with columns?

Yes. Rows and columns exchange roles, while the restriction to three cover areas stays the same.

### What makes Jellyfish different?

Jellyfish uses four starting and four cover areas. Check smaller patterns before tracking four areas at once.

## Keep practicing in Unisu

Download Unisu for iPhone or Android. Solve a puzzle or work through a guided lesson on the technique.

-   [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en)
-   [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en)

## Useful before this guide

### X-Wing

[Read more: X-Wing](https://unisu-sudoku.app/en/sudoku-techniques/x-wing/)

## Where to go next

Advanced techniques

Unisu · Oct 27, 2026

### [Jellyfish: Four lines form one pattern](https://unisu-sudoku.app/en/sudoku-techniques/jellyfish/)

A Jellyfish confines a digit’s candidates in four rows to four columns. Learn how this pattern rules out candidates outside those starting rows.

Unisu

### [Learn Sudoku: a path from rules to techniques – Unisu](https://unisu-sudoku.app/en/learn/)

Start with the rules, get comfortable with pencil marks and choose a technique for the situation you’re working on.

Download app

## Ready for a puzzle? Take a moment.

Download Unisu on your phone. Start a Sudoku or choose a lesson on the technique you want to understand.

-   [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en)
-   [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en)

![The daily challenge in Unisu](https://unisu-sudoku.app/_astro/en.C-sYR9hJ_1QwI9G.webp)

![A classic Sudoku game in Unisu](https://unisu-sudoku.app/_astro/en.C6DJ1kiE_2dRWDN.webp)