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.
A hidden single is the move you make when a unit has only one cell that can legally hold a particular digit, even if that cell could in principle hold several different digits. The cell looks ambiguous from a cell-first perspective; the digit's options are unambiguous from a unit-first perspective.
How to spot one
Pick a unit — a row, a column, or a 3×3 box. Pick a digit that hasn't been placed in that unit yet. Of the empty cells in this unit, how many could legally hold this digit? If the answer is exactly one, that cell is the home of the digit. The digit has nowhere else to go in the unit, even if the lucky cell still carries other candidates of its own.
The "hidden" in the name refers to that ambiguity at the cell. A naked single's cell screams its only candidate from the rooftop; a hidden single's cell looks indistinguishable from its neighbours until you adopt the digit-first lens.
When you'll see it
Easy puzzles solve to roughly 60% completion on naked singles before hidden singles take over. Medium puzzles need both from the start. The standard easy-to-medium workflow alternates: scan for naked singles cell by cell, place every one you see, then switch to digit-first scanning across each unit, find the hidden singles, place those, repeat. The flip between perspectives is the move.
Why the perspective matters
Almost every harder Sudoku technique is built on one of these two views or the interaction between them. Naked pairs and triples extend the cell-first lens; pointing pairs and box-line reductions extend the digit-first lens. Even the X-wing is a hidden-single argument applied to two rows or columns at once. If the perspective flip feels automatic, the rest of the technique ladder is a series of variations on a move you already make.
For a longer walk-through alongside its sibling the naked single, see The two moves that solve most easy puzzles. For the perspective shift itself, see From scanning cells to scanning units.
See also
- Naked single— A cell on the Sudoku grid that has only one legal candidate left — the simplest deduction in the game, and the one that solves most of an easy puzzle.
- 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.
- 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.
- Unit— Collective name for a row, column, or 3×3 box — the three groupings Sudoku's no-repeats rule applies to. Every cell sits in exactly three: its row, column, and box.
Read more
- The two moves that solve most easy puzzles
The naked single and the hidden single — the two foundational Sudoku moves, what each one looks like, and the perspective shift between them.
- From scanning cells to scanning units
The single biggest perspective shift between beginner and intermediate Sudoku — what it looks like, why it matters, and how to make it automatic.