Naked quad

Naked quad extends the pair/triple pattern to fours. Four cells in a unit whose combined candidate set has exactly four digits — and those four digits can be eliminated from the rest of the unit.

Quads are rare in everyday puzzles because by the time a unit has narrowed enough for a quad to appear, simpler techniques have usually fired first. But on tougher puzzles they do show up.

When to look for it

Naked quads are most often spotted in rows or columns rather than boxes, since rows and columns have more spread to host four cells with overlapping candidates.

How to apply it

1. Find four cells in the same unit, each with 2–4 candidates.
2. Check whether their combined candidates total exactly four digits.
3. Remove those four digits from every other cell in the unit.

Example