Expert

Nice Loop

When an alternating inference chain closes into a loop.

Explanation

A continuous Nice Loop alternates strong and weak links and comes back to its starting candidate. Because the loop is consistent in both directions, every weak link acts like a strong one: the other candidates in a weak-link cell, and the linked digit elsewhere in a weak-link unit, can all be removed.

Nice Loop example Sudoku board 5 4 7 2 3 2 5 8 7 1 4 7 4 2 1 5 1 7 4 2 9 4 2 5 7 2 7 6 4 7 9 5 3 6 7 4 1 5 7
An example board where Nice Loop applies

Practice tip

Start from a bivalue cell or a conjugate pair, alternate strong and weak links, and watch for the chain meeting itself again.

Example steps

  1. Chain candidates with alternating strong and weak links.
  2. Close the chain back onto its starting candidate.
  3. Remove the extra candidates around every weak link of the loop.

Try this technique on a real puzzle.

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