Hidden Singles
Short answer: a hidden single is a shape that fits only one cell in a row, column or box — so it must go there, even if that cell could hold other shapes too. It's the technique scanning naturally uncovers, and it often cracks a puzzle faster than anything else.
What a hidden single looks like
Instead of asking what fits a cell, ask where a shape can go. Choose a unit that's missing the shape, then rule out every cell where it's blocked — filled cells, and cells whose row or column already holds that shape. Sometimes just one cell is left.
That's the key difference from a naked single: the ringed cell still has two candidates (slash and star). It's not the cell that's down to one option — it's the star that's down to one cell.
How to hunt hidden singles
- Pick a shape to hunt. Choose one of the nine shapes — ideally one already placed several times, so it constrains a lot of the grid.
- Take a unit missing that shape. Look at a box, row or column that does not yet contain your chosen shape.
- Cross off the impossible cells. A cell is out if it is already filled, or if the same shape sits in a crossing row or column. Rule those cells out of the unit.
- Place it if one cell remains. If exactly one cell survives, that shape must go there — even if the cell could also hold other shapes. That is the hidden single.
Boxes are the easiest place to start, because a 3×3 box is blocked by three rows and three columns at once. This is the same move as cross-hatching — hidden singles are just the name for what scanning finds.
Hidden single vs naked single
Two sides of the same coin. A naked single is a cell with one candidate; a hidden single is a shape with one cell. When one technique stalls, switch to the other — together they solve almost every easy puzzle.
Frequently asked questions
What is a hidden single in sudoku?
A hidden single is a shape that can legally be placed in only one cell of a given row, column or box. Even if that cell still has several candidates, the shape has nowhere else to go in the unit — so it must go there.
Why is it called "hidden"?
Because the winning candidate is hidden among others in the cell. Unlike a naked single, the cell isn't down to one option — you find the move by looking at the shape's options across the whole unit, not the cell's options.
Are hidden singles harder to find than naked singles?
Not really — they're often easier, because scanning naturally finds them. Running one shape through each box (cross-hatching) is exactly how you uncover hidden singles, so many solvers spot them before they'd notice the equivalent naked single.
Which should I look for first, hidden or naked singles?
Scan for hidden singles first — they need no pencil marks. Reach for naked singles when scanning dries up and you've noted candidates.