Hidden Single
A digit with one and only one possible cell within a row, column, or box — even when that cell still has multiple candidates of its own.
Beginner sudoku technique
What it is
A hidden single appears when, inside a single unit (row, column, or box), there is only ONE empty cell that can hold a particular digit — even though that cell might still be a legal home for several other digits too. Hidden singles are harder to spot than naked singles because you're scanning by digit, not by cell. You ask: 'where can 7 go in this row?' If the answer is one cell, that cell must hold 7.
When to use it
When naked singles dry up but the puzzle is clearly still under-constrained. Sweep each row, column, and box once per digit (9 × 3 × 9 = 243 checks at the start of a puzzle, but most are dismissed instantly).
Worked example
In box 5 (the centre 3×3), the digit 4 can legally appear in R4C4, R5C5, and R6C6. Two of those cells are blocked by a 4 elsewhere in their row, so the only remaining home for 4 in box 5 is R5C5. Place 4 there — even if R5C5 still has {4, 7, 9} as candidates, only 4 has nowhere else to go.
Try it
The Sudoku247Online solver walks you through every move of any puzzle one logical step at a time, naming the technique that justifies each placement. Paste a puzzle to see this technique applied in real time.