GridWits
Hard tier Technique 13 of 21 Reference only

Naked Quad

Four cells in a row or column whose candidates combine to exactly four heights — eliminate those heights elsewhere in the line.

The solver does not attempt this technique, so Smart Hint will never suggest it. It is documented because you may still want to spot it yourself.

How it works

  1. 1Four cells whose candidates between them cover only four heights: 1, 2, 3 and 4.
  2. 2Four cells, four heights — all four are consumed inside that group, whatever order they end up in.
  3. 3Strike them from the row's remaining cell, which leaves it a 5.
  4. 4Be honest about the size, though: with one cell left over, the 5 has nowhere else in the row to go, so a Hidden Single settles it far earlier. A Naked Quad only leads in a line of eight or more blanks — the deep end of Skyscrapers+. Read this as the shape, not as the move.
1 2 2 3 3 4 1 4 2 4 5
  • Height ruled out
  • Relationship — The arrow shows which cells support the deduction.
  • Conclusion — The final arrow lands beside the proved or ruled-out height.
Next in the ladder Hidden Quad →

Four heights only fit in the same four cells of a row or column — strip those cells down to just those four.