Intermediate
Hidden triple
Three digits share a three-cell pool between them.
A hidden triple is the digit-side mirror of a naked triple. Three digits each have the same set of three candidate cells inside a unit. The cells themselves may still wear other candidates on paper, but those candidates are stale — only the three digits will end up there.
Spotting one means scanning each unit digit-by-digit and looking for three digits whose pools collapse onto the same three cells.
When the move applies
Hidden triples need a relatively narrow candidate field — they tend to appear after subsets and locked candidates have already done their work.
The procedure
- In a unit, list each digit's remaining candidate cells.
- Find three digits whose combined pools are exactly the same three cells.
- In those three cells, drop every candidate that isn't one of those three digits.