Hidden triple in Sudoku

How it works

A unit needs each of the three digits once. If the only cells that can take any of them are the same three cells, then those cells will hold the three digits and nothing else. Other notes in those cells are removed.

As with a naked triple, each digit does not have to appear in all three cells. One digit might fit in only two of them. What matters is that none of the three digits can go anywhere outside the three cells.

1. Choose a unit with many empty cells.
2. For each missing digit, note which cells it can go in.
3. Find three digits whose possible cells, combined, are exactly three cells.
4. Erase every other note from those cells.

A real example

The blue candidates are the three digits of the triple. In the highlighted unit they appear only in these three cells. The red candidates sharing those cells are removed.

How to spot them

This is one of the hardest basic patterns to see, because the three cells are crowded with other notes. Scan by digit instead of by cell: look for digits that appear only two or three times in a unit, and see whether three such digits keep to the same three cells. Units with six or more empty cells are where to look.

How often you need one

Rarely. Most of the time an easier move is available somewhere else on the board, and a hidden triple in a unit can often be read as a naked group among the unit's other cells. When a puzzle does need one, it is usually the single hardest step of the whole solve.