Lesson 7 of 8
Hidden triple
A hidden triple is three digits that can go in only three cells of a unit. Those three cells have to hold those three digits between them, so every other candidate written in them can be removed.
- Level
- Advanced
The pattern
What it does.
Like the hidden pair, this one is read from the digits rather than the cells. The cells may look busy, but the three digits you're following have nowhere else to go.
It's the last of the techniques built on sets of cells and digits, and the rarest of them. Most puzzles that reach this point can also be finished with the simpler patterns, so treat it as a fallback rather than a routine scan.
Example
A worked example.
Take one unit and three digits
Choose a row, column or box, and pick three digits that it hasn't placed.
Count the homes for each
For each of the three digits, list the unsolved cells of the unit that can still take it, judged from the grid rather than from your pencil marks.
Three cells, no more
If the three lists together name only three cells, the digits are hidden in them. A fourth cell in any one of the lists means there's no hidden triple here.
Clear the three cells
Remove every other candidate from those three cells, leaving the three digits behind. The cells are now a naked triple for the rest of the unit.
Spotting it
How to spot it.
- Three digits that each appear in only two or three cells of a unit. The overlap is the pattern.
- Units that are crowded with candidates, where the three digits in question are almost lost among them.
- A puzzle that has resisted pairs, triples and locked candidates. A hidden triple is usually the reason it hasn't moved.
Where it stops working
It removes candidates rather than placing digits, and it needs a complete candidate list to be visible at all. If you have found a naked triple in a unit you won't find a hidden triple in the same cells, because they are the same discovery read in opposite directions. Reach for it only after the simpler patterns have stopped working.
Keep going
Related topics.
New to sudoku? The rules take two minutes to read, and the first-puzzle walkthrough covers the basics that come before any of these.
Practice it in Sudoku Mori.
Hidden triple is lesson 7 in the game, with a worked example and three practice puzzles. Lesson puzzles are separate from your daily ones, and you can redo any lesson whenever you like.