Hidden Triple
A Hidden Triple is the three-cell version of the hidden pair. Three digits can go in only the same three cells of a unit — but those cells are cluttered with other candidates, so the triple is hidden. Find it, strip the clutter, and three cells collapse at once.
What is a Hidden Triple?
Look at a unit from the digits' side. If three numbers — say 2, 5 and 8 — can each only go in the same three cells, then those three cells must hold the 2, 5 and 8 between them. Every other candidate in those three cells can be deleted, turning a hidden triple into a plain naked triple.
As with the hidden pair, each cell needn't show all three digits — they just can't appear anywhere else in the unit.
The digits 2, 5 and 8 appear only in the three indigo cells. So those cells are {2,5,8} between them — delete the stray 7 and 9 from them.
Step by step
- Full pencil marks first. Hidden subsets are invisible without them.
- Track three digits. For a unit, find three numbers that between them fit in only three cells.
- Confirm the lock. None of the three may appear in a fourth cell of the unit.
- Clear the extras from those three cells — everything but the triple's own digits.
Common mistakes
- A digit leaking into a fourth cell. If any of the three can go elsewhere in the unit, it isn't a hidden triple.
- Confusing it with the naked version. Naked = three cells with three candidates; hidden = three digits in three cells. Both end the same way.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.