Hidden Pairs and Triples in Sudoku, Explained

Naked versus hidden

A naked pair is easy to picture. Two cells in the same unit share exactly the same two candidates and nothing else, so those two values are locked to those cells. A hidden pair is the mirror image. Two values appear as candidates in only two cells of a unit, but those cells may carry other candidates too. The pair is there, just hidden among the clutter.

Why the hidden version matters

When you find a hidden pair, you learn that those two values must occupy those two cells, in some order. That means every other candidate in those two cells can be erased. Suddenly two crowded cells become a clean pair, and that cleanup often triggers a run of new placements. Hidden pairs reach eliminations that scanning and naked pairs simply cannot.

How to spot a hidden pair

1. Pick a unit, a row, column or box, and list which cells could hold each value.
2. Look for two values that can go in only the same two cells of that unit.
3. You have found a hidden pair. Those two cells belong to those two values.
4. Erase all other candidates from those two cells.
5. Rescan the unit, since the newly cleaned cells often reveal more moves.

Hidden triples work the same way

A hidden triple is the same idea with three values and three cells. If three values can only appear in the same three cells of a unit, those cells belong to those three values, and every other candidate in them can go. Triples are harder to see because the three values need not all appear in all three cells, only be confined to them. But the payoff is the same clean up, and spotting one is deeply satisfying.

A tip for finding them

Hidden pairs and triples are easiest to find when you focus on values rather than cells. Instead of asking what can go in this cell, ask where can this value go in this unit. When two or three values keep pointing at the same small cluster of cells, you have likely found a hidden set. Accurate candidate notes are essential, so keep them tidy.

These techniques turn a stuck hard board into a solvable one, and they reward patient looking. The Extreme puzzles in Shapedoku are a good place to practice, since they are built to need exactly this kind of careful candidate work, always with a single logical solution waiting to be found.