GridWits
Hard tier Technique 9 of 11 Used by Smart Hint

Duplicate Trap

Keeping this cell would reject every other copy of its number in its row and column at once — and that burst of rejections would seal part of the board off from the rest. So the cell is rejected instead.

How it works

  1. 1Suppose the corner 7 were kept. Its number can't repeat among kept cells in its row or column, so the 7 beside it AND the 7 below it would both have to be rejected, immediately.
  2. 2That leaves the corner surrounded by rejected cells on both its sides — cut off from every other kept cell, which the connection rule forbids.
  3. 3So the corner can't be kept: reject it.
7 7 1 2 7 3 4 5 2 1 6 8 4 6 9 1
  • Rejected
  • Cells that establish the pattern
  • Where the conclusion lands
Next in the ladder Rejection Chain →

Rejecting this cell would keep all of its neighbors, which rejects every copy of their numbers, which keeps more neighbors — a ripple followed as far as it forces. If it ends with a broken rule — most often two kept copies of one number in a line, or a piece of the board sealed off — the first cell can't be rejected: it's kept.