The intuition
Sudoku has nine of every digit. Every row, column and 3×3 box must contain each digit once. So if you pick a digit and a single unit, and all but one of the unit's empty cells are forbidden from holding that digit, the survivor must take it. A Hidden Single is the cleanest expression of that uniqueness rule — and it's usually invisible until you actively scan unit by unit.
When you'll use it
Hidden Singles appear constantly in Easy and Medium puzzles and remain the workhorse technique throughout Hard. After Naked Singles, this is the single most important technique to internalize: most boards break open the moment you commit to a disciplined Hidden-Single scan of every row, every column, and every box for digits 1 through 9.
How to spot a Hidden Single
- Pick a digit, say 7.
- Look at one unit (a row, column or 3×3 box).
- Cross out every cell in that unit where 7 cannot go because of a 7 elsewhere in its row, column or box.
- If exactly one cell survives — that's a Hidden Single for 7.
- Repeat for the next digit in the same unit, then move to the next unit.
Why the Hidden Single logic works
The Sudoku constraint says every digit 1–9 must appear exactly once in every row, every column, and every 3×3 box. So if 7 is missing from row 4, it MUST land in row 4 somewhere. If you can prove only one cell in row 4 can legally accept a 7, the puzzle has no other choice — that cell is forced. The candidate may not be visually obvious because the cell still has many other possible digits, which is why it is called 'hidden': the single is hiding inside a larger candidate list.
A worked Hidden Single example
In row 4, suppose the digit 9 is already pinned in columns 1, 2, 3, 5, 6, 7, 8 and 9 by 9s elsewhere on the board. Then the only legal position for 9 in row 4 is column 4 — a Hidden Single, even though R4C4 still has many other candidates on paper.
Walkthrough
The eight green-tinted 9s on the board each remove the digit 9 from a column or a box that intersects row 4. R1C1 = 9 kills 9 in column 1. R2C5 = 9 kills column 5. R3C9 = 9 kills column 9.
R5C2 = 9 doesn't just kill column 2 — it also kills the entire 3×3 box that contains R4C1, R4C2 and R4C3. R6C7 = 9 likewise kills column 7 and the box containing R4C7, R4C8 and R4C9.
Add R7C6 = 9, R8C3 = 9 and R9C8 = 9 and every cell of row 4 except R4C4 has a 9 forbidden in at least one of its row, column or box. The Hidden Single survives.
Notice that R4C4 itself probably still has several candidates on a real candidate-pencil-mark grid. The Hidden Single is hidden precisely because the digit 9 isn't the only candidate there — it's the only candidate for 9 anywhere in row 4.
How to drill this technique
Pick any solved puzzle, erase every cell of one row, then put back one digit at a time and try to spot the Hidden Single before it goes in. Five minutes of this on a beach towel of squared paper retrains your eyes faster than any video.
Common mistakes
- Scanning a single cell instead of a single unit-and-a-single-digit pair.
- Forgetting to also check the 3×3 box (not just rows and columns).
- Confusing Hidden Single with Naked Single — a Hidden Single's cell may still have many candidates, but the unit only has one home for the digit.
- Trying to spot Hidden Singles for all nine digits at once. Always scan one digit through one unit.
FAQ
What's the difference between a Naked Single and a Hidden Single in Sudoku?
A Naked Single is a cell that has only one candidate digit left. A Hidden Single is a digit that has only one candidate cell left inside its row, column or box — even though that cell may have many candidates itself. Naked Singles are easier to see; Hidden Singles are what unlocks the rest of an Easy or Medium puzzle.
Is the Hidden Single technique enough to solve an Easy Sudoku?
Almost. A well-tuned Easy puzzle is solvable using only Naked Singles plus Hidden Singles. If you finish those scans and the board is still incomplete, you're either at a Medium puzzle or you missed a Hidden Single somewhere.
How fast should I be at spotting Hidden Singles?
After about a week of daily practice most players can scan all nine units for one chosen digit in well under a minute. The bottleneck is rarely speed of recognition — it's the discipline to stay on one digit and one unit until you're sure.
Why is it called a 'hidden' single in Sudoku?
Because the digit is hiding inside a longer candidate list. The cell that will receive the digit may still display six or seven other candidates on a pencil-marked grid — but the unit as a whole has only one home left for the digit in question. The single is hidden, not the cell.
Do Hidden Singles work in 3×3 boxes as well as rows and columns?
Yes. A Hidden Single can occur in any of the three unit types — row, column, or 3×3 box. Box scans are particularly powerful because they're the most constrained unit on the board, and beginners often forget to do them.
What to learn next
Once Hidden Single feels natural, move on to Naked Pair — it builds directly on the candidate-tracking habits you've just developed. Or revisit the full Sudoku techniques library to see the full progression from beginner to advanced.