A hidden triple is three digits that, within one row, column, or box, can only go in the same three cells. Those three cells must hold those three digits, so every other candidate in them can be erased. It is the three-digit version of a hidden pair, and it is one of the harder basic patterns to see, because the three cells are usually full of other candidates.
The rule
In one group, look for three digits whose possible positions, combined, cover exactly three cells. Each digit can appear in two or three of those cells, but none of the three digits may appear anywhere else in the group. When that holds:
- The three cells must contain the three digits.
- Erase every other candidate from those three cells.
- Leave the rest of the group alone. A hidden triple only cleans up its own cells.
Worked example
Row 2 is missing 1, 2, 3, 4, 5, and 6. The six empty cells have these candidates:
- R2C1: 1, 2
- R2C2: 1, 4, 5
- R2C4: 2, 3
- R2C5: 2, 4, 6
- R2C7: 1, 3
- R2C8: 3, 5, 6
Count by digit. 4 fits in C2 and C5. 5 fits in C2 and C8. 6 fits in C5 and C8. Together, 4, 5, and 6 only reach three cells: C2, C5, and C8. That is a hidden triple, and like the trickiest naked triples, no cell holds all three digits.
Those three cells must take 4, 5, and 6, so erase everything else from them. R2C2 loses 1 and becomes {4, 5}. R2C5 loses 2 and becomes {4, 6}. R2C8 loses 3 and becomes {5, 6}. Nothing else in the row changes.
Look at the other three cells: {1, 2}, {2, 3}, {1, 3}. They form a naked triple on 1, 2, and 3. That is not a coincidence, and it leads to the best shortcut for finding hidden triples.
A cleaner way to find them
Searching for hidden triples directly is slow. Two shortcuts make it practical:
Look for the naked set in the same group
Every hidden triple in a group has a matching naked set in the group's other cells. If a row has six empty cells and three of them form a hidden triple, the other three form a naked triple. Naked sets are often easier to see, so check for those first. Applying either one produces the same eliminations.
Build a digit map for the group
For the group you are stuck on, write each missing digit next to the cells it can reach, like "1: C1 C4 C7". Digits with two or three positions are the only ones that can be part of a hidden triple. Look for three of them whose positions merge to three cells. On paper this takes a minute. In an app with good notes it takes less.
Drill hidden triples in the Dojo
Sudoku Royale's Dojo has a short lesson and a drill for this pattern: board after board where it is the next move. Your record is your fastest run of correct answers in a row, and you can see how it compares with other players. Open Hidden Triple on your iPhone, or read how the Dojo works.
Get Sudoku Royale: Free on iOSWhere hidden triples fit
Hidden triples show up on hard and expert puzzles, after singles, locked candidates, and pairs have run out. If you are still finding pointing pairs you have not used, use those first. For the full order, see our sudoku techniques guide, and for the next step up, the X-Wing.
Frequently Asked Questions
What is a hidden triple in sudoku?
A hidden triple is three digits that can only go in the same three cells of a row, column, or box. Those cells must hold those digits, so all other candidates can be removed from the three cells.
How do I find hidden triples?
For one group, list where each missing digit can go. Look for three digits whose positions combine to exactly three cells. It is often faster to look for a naked set among the other cells in the group, since every hidden triple has one.
Are hidden triples common?
No. They appear mostly on hard and expert puzzles, and many of those can be solved with the matching naked set instead.