GridWits
Easy tier Technique 4 of 11 Used by Smart Hint

Duplicate of a Kept Cell

Once a cell is kept, its number can't repeat among kept cells — every other copy in that row and column is rejected.

How it works

  1. 1The top-left 5 is already known to be kept.
  2. 2Its number can't repeat among kept cells in the same row or column, so the other 5 in the top row must be rejected.
5 1 5 2 6 3 4 8 9
  • Kept
  • Rejected
  • Cells that establish the pattern
  • Where the conclusion lands
Next in the ladder Don't Trap a Cell →

A cell with only one free neighbour left would be sealed off if that neighbour were rejected too — so the neighbour is kept.