Naked Quad
A Naked Quad is the widest member of the naked subset family — after the pair and triple comes the quad. Four cells in a unit that between them use only four candidates lock those digits in, clearing them from everywhere else in the unit.
What is a Naked Quad?
When four cells in the same unit together contain only four candidates — no others — those four digits must fill those four cells in some order. So they can be removed from every other cell in the unit. As always with subsets, each cell can show any mix of the four; they simply mustn't use a fifth digit between them.
Four cells use only {1, 4, 6, 9}. The green cell held {4, 5}; removing the 4 leaves a naked single — 5.
Spotting one
- Look for cells with two, three or four candidates clustered in one unit.
- Add up the distinct digits. If four cells use exactly four different candidates, that's a naked quad.
- Eliminate those four digits from every other cell in the unit.
Naked quads are rarer than pairs and triples, and often a hidden single or a simpler subset will crack the grid first. But when nothing else moves, counting a quad can be the key.
Common mistakes
- A fifth digit sneaks in. Four cells using five candidates between them is not a quad.
- Over-hunting. Check for singles, pairs and triples first — they're faster and usually enough.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.