---
title: "X-Chain: Sudoku Technique #24 | Sudoku Flow"
description: "X-Chain: In an X-Chain, strong and weak links on a single number alternate, starting and ending with strong links, so at least one of the two chain ends must…"
url: https://sudokuflow.com/lessons/xChain/
language: en
---

[Sudoku technique lessons](https://sudokuflow.com/lessons/) / X-Chain

Lesson 24

# X-Chain

## The rule

In an X-Chain, strong and weak links on a single number alternate, starting and ending with strong links, so at least one of the two chain ends must hold that number, and any cell that sees both ends loses it.

## Step by step

1. These 6 cells all hold the candidate 7 and form a chain. The first link, the last link, and every second link between them are strong links: the two cells in each strong link are the only spots for 7 in one row, column, or box. If either end cell were not 7, the links would force the other end to be 7.
2. At least one of the two end cells must be 7, so we can remove the candidate 7 from any other cell that sees both ends.

[Read the full lesson, with more examples →](https://sudoku.rubigogames.com/techniques/x-chain.html)

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 X-Chain applies. Find the step yourself, or tap Hint and the app's own hint walks you through it.

X-Chain practice board:

```text
2  .  .  | .  .  3  | 1* 4  .
.  5  3* | 4* .  6  | .  7  .
.  .  .  | 2* .  8  | 9  .  3
---------+----------+---------
.  .  .  | 3* 2  9  | .  .  .
3  .  1  | 8* .  .  | 7  .  9*
.  .  9  | 7* 6  1* | .  3  .
---------+----------+---------
8  .  .  | .  3  7* | 6  .  4
.  .  7  | 6* .  .  | 3* 9  .
.  3  .  | 1  .  .  | 5  .  7*
```
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.