Hidden Quad
Hard
Used by Smart Hint
Four digits confined to the same four cells in a unit — strip those cells down to just those four digits.
- Cells that establish the pattern
- Candidate ruled out
How it works
- Digits 2, 4, 6, and 9 only appear as candidates in G7, H7, I8, and G9 within this box — a Hidden Quad.
- Strip every other candidate from those four cells — G7 loses its 5 and I8 loses its 7 — since 2, 4, 6, and 9 must occupy exactly those spots.