Hidden single

A hidden single happens when a digit can only go in one place within a row, column, or box — even if that cell still has other candidates on paper. The digit is 'hidden' because the cell itself has more options, but the unit forces the placement.

Hidden singles are usually easier to spot than naked singles because you scan a digit across a unit rather than collecting candidates one cell at a time. Pick a digit, look across a row/column/box, and see how many empty cells could hold it.

When to look for it

Hidden singles are the workhorse of beginner and intermediate solving. After each placement, scan the digit you just placed across the rest of the grid — and pick neighbor digits whose placements you can predict from peer constraints.

How to apply it

1. Pick a digit (say 7) and a unit (a row, column, or box).
2. Cross out cells where 7 already appears in a peer row, column, or box.
3. If exactly one cell in the unit remains, place 7 there.

Example