Hidden Single
A digit that fits in only one cell of a row, column or box — even though that cell looks like it has several options.
What it is
A hidden single occurs when a digit can legally go in just one cell within a unit. The cell itself may still show three or four candidates, which is why the single is “hidden”: nothing about the cell stands out. The logic runs through the digit instead. Every unit must contain each digit exactly once, so if only one cell can take the 8, the 8 goes there regardless of what else that cell might have held.
How to spot it
Pick a digit and a unit, then check which cells in that unit could take it. Cross out every cell blocked by the same digit in its row, column or box. If a single cell survives, that is your hidden single. Working digit by digit through the boxes — 1 through 9, box by box — finds most of them quickly.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 3: The starting position
- Pattern cells
- 3/3
- Digits
- 8
- Eliminations
- —
Press a marked square to point it out on the board above.
Why it matters
Hidden singles are the workhorse of easy and medium puzzles, and they are the technique beginners most often overlook because they only look at cells with few candidates. Learning to scan by digit rather than by cell roughly doubles the number of moves you can find without notes.
Practise now
Solve a puzzle where this technique decides the game
Frequently asked questions
How is a hidden single hidden?
The cell shows several candidates, so it does not look special. The insight comes from the digit's perspective: within that row, column or box the digit has nowhere else to go. Once you place it, the cell's other candidates simply vanish.
Which unit should I check first?
Boxes usually pay off fastest, because a box is compact and blocking lines are easy to see. Many solvers scan boxes digit by digit, then rows and columns when the boxes run dry.
Can a cell be both a naked and a hidden single?
Yes. If a cell has exactly one candidate and that digit also fits nowhere else in the unit, both descriptions apply. It makes no difference to the move — the digit still goes in.
Put it to use
Where you will meet it
This pattern is the backbone of easy grids. Start one and you will use it within a few moves.
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