Hidden triple

A hidden triple is the mirror of a naked triple. Three digits all share the same three-cell pool inside a unit — even though the cells themselves may have more candidates listed. Because those three digits must occupy those three cells, no other digits can.

Like hidden pairs, hidden triples are spotted by scanning digits across a unit and counting placements.

When to look for it

After a few placements have narrowed the candidate field, scan each unit's digit-by-digit placement list and look for triples whose pools overlap.

How to apply it

1. In a unit, list each digit's candidate cells.
2. Find three digits whose combined candidate cells are exactly the same three cells.
3. Strip all other candidates from those three cells.

Example