Intermediate
Naked quad
Four cells share a four-digit pool between them.
Naked quads are pairs and triples scaled to four. Four cells in a unit, combined candidate set of exactly four digits — and those four digits leave every other cell in the unit.
Quads are rare in practice. By the time four cells have narrowed enough for a quad to be visible, simpler patterns have usually already cleared the unit. They do show up on tough puzzles, though.
When the move applies
Rows and columns host quads more often than boxes do — they have the room to spread four cells with overlapping candidate sets.
The procedure
- Find four cells in one unit, each with 2 to 4 candidates.
- Verify the union of their candidate sets is exactly four digits.
- Remove those four digits from every other cell in the unit.