---
title: "Box/Line Reduction: Sudoku Technique #7 | Sudoku Flow"
description: "Box/Line Reduction: If a number on a line only fits inside one box, that candidate is removed from the rest of that box outside the line."
url: https://sudokuflow.com/lessons/boxLineReduction/
language: en
---

[Sudoku technique lessons](https://sudokuflow.com/lessons/) / Box/Line Reduction

Lesson 7

# Box/Line Reduction

## The rule

If a number on a line only fits inside one box, that candidate is removed from the rest of that box outside the line.

## Step by step

1. In this row, every cell where 7 can still go lies inside the same box.
2. So we can remove the candidate 7 wherever it appears in that box outside that row.

[Read the full lesson, with more examples →](https://sudoku.rubigogames.com/techniques/box-line-reduction.html) [Watch on YouTube](https://youtube.com/shorts/OQbuYQ3LQTY)

In our app, when this technique is the next step, the hint names it and shows you why, step by step. No ads, free to download.

[App Store iPhone, iPad, Mac](https://apps.apple.com/app/apple-store/id6502922368?pt=125379530&ct=sudokuflow_cta&mt=8) [Google Play Android](https://play.google.com/store/apps/details?id=com.rubigogames.sudoku&referrer=utm_source%3Dsudokuflow%26utm_medium%3Dcta%26utm_campaign%3Dlesson)

## Try it on more boards

Each board is a real puzzle from our app, at a moment where Box/Line Reduction applies. Find the step yourself, or tap Hint and the app's own hint walks you through it.

Box/Line Reduction practice board:

```text
2  .  1  | 8  .  4* | 6  .  .
.  .  .  | 6* .  5  | .  .  1*
6  .  .  | 2  1  9  | .  8  .
---------+----------+---------
.  .  .  | 3  .  7  | 1  6* .
4  .  .  | 9* 6* 1* | 8  7  2
1* 7* 6  | .  8* 2* | .  .  .
---------+----------+---------
8  1  .  | .  .  6* | .  3  9
.  6* .  | .  .  3* | 4  1  8*
.  .  .  | 1* 9  8* | .  .  6
```
Digits marked * were filled in by a player; the others were printed with the puzzle.

![Sudoku app icon](https://sudokuflow.com/assets/app-icon.png)

## Want more boards like this?

We also make a Sudoku app for iPhone, iPad, Mac and Android. There are no ads in it at all, it works offline, and when you ask for a hint it shows you the technique instead of just filling in a number.

[App Store iPhone, iPad, Mac](https://apps.apple.com/app/apple-store/id6502922368?pt=125379530&ct=sudokuflow_web&mt=8) [Google Play Android](https://play.google.com/store/apps/details?id=com.rubigogames.sudoku&referrer=utm_source%3Dsudokuflow%26utm_medium%3Dweb%26utm_campaign%3Dsite)

Sudoku Flow is made by Rubigo Games, the developer of the Sudoku app recommended on this site.