Naked Quadruple
Working with Candidates9 / 32Four cells splitting four candidates among themselves.
The subset family's largest member: four cells of a unit whose candidates, pooled together, contain only four distinct digits. Each cell may show two, three or all four of them — the pool is what counts.
Those four digits are settled: one per cell, in some arrangement. Every other cell of the unit can drop them. Quadruples hide well because four scattered, differently-marked cells rarely look related.
See it in action
Step 1 of 4
The setup
Units with many small-candidate cells are the hunting ground. Combine four cells with two to four candidates each and check whether their pooled candidates stop at four digits. Step through this real position to see it happen.
Practice
Find the naked quadruple and remove its four digits from the rest of the unit.
Drill 1 of 29Tap a cell, then the candidate digit you want to remove.