What it is
A hidden single occurs when a digit has exactly one legal position within a row, column or box — even if that cell could still hold other digits. Because the digit must appear somewhere in the unit, that cell is decided.
How to spot it
Pick one digit and one unit. Mentally cross out every cell the digit cannot occupy (blocked by the same digit in crossing rows and columns, or already filled). If a single cell survives, the digit belongs there.
Interactive example
Use Next to walk through the deduction on the board.
Step by step
- Four 7s are already on the board.
- Where can 7 go in the top-left box?
- The 7s in the first and third rows block the top and bottom rows of the box.
- The 7s in the first and third columns block the left and right cells of the middle row — and one of them already holds a 9.
- One cell remains. Even though it could hold other digits, 7 has nowhere else to go — a hidden single.
Common mistake: Forgetting that the target cell may show several pencil marks. Beginners skip a valid hidden single because the cell "still has other options" — the point is the digit has no other options.
Put it into practice
Examples on this page are validated with the Play Sudoku Daily puzzle engine.