Hidden pair
Two digits whose only possible cells inside a unit are the same two cells — even if those cells still show other candidates. The digit-first sibling of the naked pair.
A hidden pair is two digits that, inside a single unit, can only legally go in the same two cells — even if those cells still show other candidates on top. Once you've spotted it, every other candidate in those two cells can be eliminated, because the two digits have no choice but to occupy them.
How to spot one
Pick a unit. Look at digits that haven't been placed yet. For each digit, count its possible cells inside this unit. A hidden pair is when two different digits both have exactly the same two possible cells.
Worked example: in row 4, the digit 3 can only go in cells (4, 2) and (4, 7); the digit 6 can only go in cells (4, 2) and (4, 7) as well. Those two cells must hold the 3 and the 6 between them — so any other candidate currently pencil-marked in either cell (a 5, an 8, a 9) gets ruled out.
When you'll see it
Hidden pairs are quieter than naked pairs — the cells often look busy with extra candidates, so the pattern doesn't jump out the way two cells with matching pencil marks does. The trick is to lead with the digit-first count, not the cell-first scan. On medium and harder puzzles, a hidden pair is often the move that unblocks a stuck unit; a naked pair tends to be the move that follows.
Why both shapes exist
Naked and hidden pairs are the same logical claim from two angles. A naked pair says "these two cells contain these two digits and nothing else." A hidden pair says "these two digits live in these two cells and nowhere else in the unit." The two are duals — every naked pair has a hidden pair "shadow" in the unit's other digits, though the shadow is only useful when its cells aren't already collapsed.
For a longer take on the symmetry between the two and how to scan for both, see Naked and hidden pairs and triples.
See also
- Naked pair— Two cells in the same unit whose candidate sets are identical and contain exactly two digits. Together they claim those digits across that unit and rule them out elsewhere.
- Hidden single— A digit with only one possible cell within a unit (row, column, or 3×3 box) — even if that cell could legally hold other digits. The unit-first sibling of the naked single.
- Hidden triple— Three digits whose only possible cells inside a unit are the same three cells — even if those cells still show other candidates. The digit-first sibling of the naked triple.
- Candidate— A digit (1–9) a cell could still legally hold — one not yet ruled out by anything in its row, column, or 3×3 box. Every empty cell has between one and nine.
Read more
- Naked and hidden pairs and triples
The mid-level extension of singles — how pairs and triples work, when each shows up, and the perspective shift that surfaces them.