Sudoku FlowSudoku technique 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
- 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.
- 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.
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.
Want more boards like this?
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Sudoku Flow is made by Rubigo Games, the developer of the Sudoku app recommended on this site.