Summary: After the 9×9 rules, the next two scans that place a digit without guessing are Naked Singles and Hidden Singles. A Naked Single is a cell that has only one legal digit left. A Hidden Single is a digit that has only one legal cell left in a row, column, or 3×3 box. Each section below works one mini-grid.
Start from the rules, then scan
If a row, column, or box still has two or more empties, “fill the last hole” is not enough. You need a scan that either asks what can this cell be? or where can this digit go?
Those two questions are the whole page. For the 9×9 rules and the first fills on a nearly-full box, see How Sudoku Works.
Count rows down from the top and columns left to right. A printed digit stays put. Blank cells are empty. Each figure shows the board twice: on the left the scan is set up, with the cell it resolves tinted; on the right that one digit is written in. Every other cell is left blank on purpose — the figure gives away nothing but the placement.
Naked Single: one cell, one legal digit
A Naked Single is an empty cell whose row, column, and 3×3 box together already show eight of the nine digits. The leftover digit is the only one that can go in that cell.
You find it by staring at one cell. List every digit that already appears in its row, its column, or its box. If eight digits are taken, write the ninth. You do not need to ask where else that digit could go.
Worked example
Look at the tinted cell in row 2, column 2 — the center of the top-left box.
Digits that already see that cell:
- Top-left box: 1, 2, 4, 6, 8
- Row 2 (outside the box): 3 at column 5, 7 at column 8
- Column 2 (outside the box): 5 at row 6
That is 1, 2, 3, 4, 5, 6, 7, and 8. The tinted cell cannot take any of them. Write 9.
The same 9 might still be legal in another empty of that box — this scan never checked. It only proved that this cell has no other digit left. That is why the single is called naked: the leftover is sitting on the cell itself.
Hidden Single: one digit, one legal cell in a house
A Hidden Single is a digit that can go in only one empty cell of a single house — one row, one column, or one 3×3 box — even when that cell still looks like it could take several digits.
You find it by staring at one digit in one house. Pick a house. Pick a missing digit. Cross off every empty that already sees that digit in its other row, column, or box. If one empty remains, write the digit there.
The cell is not a Naked Single. It still has other candidates. The digit is hidden among them until you ask where that digit is allowed to live.
Worked example
Look at the center box (rows 4–6, columns 4–6). Ask where 4 can go in that box. The tinted cell is row 5, column 5.
The center box already shows 1, 2, 6, and 8. Its five empties are row 4 column 4, row 4 column 6, the tinted cell, row 6 column 4, and row 6 column 6.
- Row 4 already has a 4 at column 1, so neither empty in row 4 of this box can be 4.
- Row 6 already has a 4 at column 8, so neither empty in row 6 of this box can be 4.
The tinted cell is the only empty in the center box that does not already see a 4. Write 4 there.
That cell is not a Naked Single. It still sees only 1, 2, 6, and 8, so 3, 5, 7, and 9 remain possible if you only ask “what can this cell be?” The 4 appears when you ask “where can 4 go in this box?”
Which question are you asking?
- Naked Single — What digits are still legal in this cell? You have a placement when eight digits are already in its row, column, or box.
- Hidden Single — Where can this digit go in this house? You have a placement when every other empty in that row, column, or box already sees that digit.
A house with one empty cell is both at once: the leftover digit is the only candidate in that cell, and it has only one home in that house. How Sudoku Works starts there, on a nearly-full box. Use this page once two or more empties remain and you need a tighter scan.
Scan Naked Singles first — they look at one cell. When a cell still has several candidates, switch to Hidden Singles and walk the missing digits house by house.
Do not guess. If two cells in a house still accept the same digit, that digit is not a Hidden Single yet. Leave it and look at another house.
Practice on a fresh 9×9
Print a Very Easy or Easy 9×9, cover the solution, and place every Naked Single you can see before you start hunting Hidden Singles. Those two levels are the right practice set for this page.
Puzzle Maker Pro Sudoku prints classic 9×9 grids with a matching answer key. The generate walkthrough is Getting Started with Puzzle Maker Pro Sudoku. The free demo generates one page at a time if you want to try a board first.
FAQ
What is a Naked Single in Sudoku?
An empty cell whose row, column, and 3×3 box already contain eight different digits. The ninth digit is the only one that can go in that cell.
What is a Hidden Single in Sudoku?
A digit that has only one legal empty cell left in a single row, column, or 3×3 box. That cell may still look like it could hold other digits until you ask where the chosen digit is allowed to go.
Do I need pencil marks?
No. Pencil marks make Hidden Singles faster because you can see a digit that appears in only one cell of a house. You can also find the same placement by walking a missing digit across the empties of that house, as in the center-box example.
Which scan should I do first?
Naked Singles. They look at one cell. If that cell still has more than one legal digit, pick a house and hunt Hidden Singles for the digits it still needs.
Can a 9×9 be finished with only these two techniques?
Yes. Many Easy pages never need more. When two cells in a house still accept the same digit, stop — that digit is not a Hidden Single yet, and guessing is not a technique. Later pages cover the scans that remove candidates without placing a digit.
Where do I learn the 9×9 rules first?
How Sudoku Works — rows, columns, 3×3 boxes, and the first fills on a nearly-full box. Dot, Hyper, and Multidoku are sections on that same page.