Hidden Triple
Three digits fit only the same three cells in a unit.
A hidden triple in Sudoku is three digits that, between them, can only be placed in the same three cells of one row, column or box — even though each of those cells still displays other candidates that hide the trio from view. Solving a hidden triple sudoku position means recognising that lock and deleting every extra candidate from the three cells, because they are now reserved for the three digits. It is the concealed cousin of the naked triple: the same three-cells-for-three-digits pigeonhole, but found by tracking where digits can go rather than by reading three short candidate lists.
Worked example
Here, 1, 2 and 3 have already been placed in row 2 and in row 3, which bars all three of those digits from row 1 everywhere in columns 4 to 9. That leaves R1C1, R1C2 and R1C3 as the only cells in row 1 that can still hold a 1, a 2 or a 3, so {1,2,3} is hidden inside those three cells among the 4-to-9 candidates they also carry. Because three digits are pinned to exactly three cells, every other candidate — 4, 5, 6, 7, 8 and 9 — is struck out of R1C1, R1C2 and R1C3.
When to use it
Reach for a hidden triple once basic singles and the easier-to-see naked pairs and triples have dried up, and a row, column or box still won't crack. Spot it by choosing one unit and asking, digit by digit, "where in this unit can this number still go?" — if three different digits are each confined to the same three cells (in any arrangement), you have a hidden triple, even when those cells look crowded with unrelated candidates.
How to use it
- Track three digits. Find three digits in a unit whose possible positions fall within the same three cells.
- Confirm the hidden triple. Those three digits are locked to those three cells, regardless of any other candidates listed there.
- Clean the cells. Remove all other candidates from the three cells.
Frequently asked questions
What is a hidden triple in Sudoku?
A hidden triple is three digits that can only appear in three cells of a single row, column or box, so those cells are reserved for the trio. In the worked board, 1, 2 and 3 are blocked from row 1 across columns 4-9, leaving R1C1, R1C2 and R1C3 as their only homes; that lets you delete every other candidate (4-9) from those three cells. It is 'hidden' because the three cells still show extra candidates that mask the trio.
How do I spot a hidden triple on the grid?
Pick one unit and, for each digit 1-9, note which cells in that unit could still take it. When three digits' possible positions collapse onto the same three cells — as 1, 2 and 3 do onto R1C1, R1C2 and R1C3 in this example — you have found a hidden triple. Note that not every digit needs to be possible in all three cells; the pattern only requires that those three digits appear nowhere else in the unit.
What is the difference between a hidden triple and a naked triple?
A naked triple is three cells that between them contain only three candidates, so you read it straight from short lists and eliminate those digits from the rest of the unit. A hidden triple works the other way round: the three cells still carry extra candidates, and you find the trio by noticing that three digits are confined to those cells. The naked triple clears candidates outside its cells; the hidden triple clears the surplus candidates inside them — here, stripping 4-9 from R1C1, R1C2 and R1C3.