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XY-Chain

When two-candidate cells form a forcing chain.

Explanation

An XY-Chain links bivalue cells where neighboring cells see each other and share one candidate. If the chain endpoints contain the same digit, one endpoint must hold that digit, so common peers can remove it.

XY-Chain example Sudoku board 4 6 7 5 8 9 1 3 6 2 7 8 9 1 1
An example board where XY-Chain applies

Practice tip

Start with bivalue cells. Follow shared candidates until the chain returns to the same endpoint digit.

Example steps

  1. Find cells with exactly two candidates.
  2. Link peer cells that share one candidate.
  3. When endpoints share a digit, remove it from cells seeing both ends.

Try this technique on a real puzzle.

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