What is a naked single?
A naked single is an empty cell that has exactly one possible number left. Every other number already appears in the cell’s row, column or 3 × 3 box, so the last one must go there. It is also called a sole candidate or single candidate, and it is the most common move in every Sudoku.
In Marble Sudoku the numbers 1 to 9 are nine marble colors, so you look for the one color an empty hole is missing. The logic is exactly the same.
Find one in 3 checks
- 1Row. Cross out every number already in the cell’s row.
- 2Column. Cross out every number already in its column.
- 3Box. Cross out every number already in its 3 × 3 box.
One number left? Place it. Two or more? Try another cell.
TRY IT · GUIDED EXAMPLE
Solve the ringed cell yourself
Tap the marbles in the glowing group. Every color you tap gets crossed off the list below the board.
This is the real board from Marble Sudoku. The nine marble colors stand for the numbers 1 to 9.
- Row
- Column
- Box
- Place
STEP 1 OF 3
Check the row
Tap each marble in the glowing row.
CANDIDATES 9 of 9 left
Read the example in words
The ringed cell is in the middle of the board (row 5, column 5). Its row already holds Purple, Green, Yellow and Pink, which leaves 5 colors. Its column holds Red and Sky, which leaves 3. Its 3 × 3 box adds Orange and Teal (Red, Green and Yellow are in the box too, but they are already crossed out). Eight colors are used, so Navy is the only candidate. That makes it a naked single. With numbers turned on, the same cell is an 8.
YOUR TURN · PRACTICE BOARD
Find the naked singles
Tap an empty hole, then tap the color that belongs there. Only proven moves count: the cell must have exactly one candidate. Every marble you place can unlock new naked singles nearby.
PICK A COLOR
Pick an empty hole to start.
Why a naked single always works
Each row, column and 3 × 3 box must contain all nine numbers exactly once. So a number that already sits anywhere in the cell’s row, column or box can never go in that cell. The cell still needs one of the nine numbers. When eight of them are ruled out, only one choice is left. It is a proof, not a guess.
The name comes from candidate notes: once you write down what each cell can still hold, a naked single is a cell whose single note sits there in plain sight. A hidden single, by contrast, is buried among other notes.
How to spot naked singles faster
- Start where it is crowded. A cell whose row and box are nearly full has the fewest candidates.
- Count to eight. List the different numbers you see in the row, column and box together. If you reach eight, the missing one is your answer.
- Follow the chain. After each placement, re-check the empty cells in the same row, column and box. New naked singles often appear right next to the last one.
- Use notes on harder grids. When candidate notes are up to date, any cell with a single note is a naked single.
Naked single vs. hidden single
| Naked single | Hidden single | |
|---|---|---|
| Question you ask | Which numbers can go in this cell? | Where can this number go in this row, column or box? |
| You look at | One cell and its row, column and box | One group and all of its empty cells |
| It is found when | The cell has one candidate left | The number has one possible cell left |
| Other candidates? | None in that cell | The cell may still have others |
Common mistakes
- Skipping the box. Row and column alone often leave two or three options. The box check is usually what makes the single appear.
- Counting a number twice. A number that is in both the row and the box only rules out one candidate. Count different numbers, not marbles.
- Trusting old notes. After you place a number, remove it from the notes of every cell in the same row, column and box.
- Placing a “probably”. If two candidates are left, it is not a naked single yet. Look for another cell or a hidden single instead.
Naked singles FAQ
What is a naked single in Sudoku?
A naked single is an empty cell that has exactly one possible number left. The other eight numbers already appear in the cell’s row, column or 3 × 3 box, so the remaining number must go there.
Is a naked single the same as a sole candidate?
Yes. Naked single, sole candidate and single candidate are different names for the same technique: a cell with only one candidate.
What is the difference between a naked single and a hidden single?
A naked single looks at one cell and finds that only one number fits it. A hidden single looks at one row, column or box and finds that a number fits in only one of its cells, even if that cell has other candidates.
Do I need pencil marks to find naked singles?
No. On easy puzzles you can find naked singles by scanning the row, column and box in your head. On harder puzzles, candidate notes (pencil marks) make naked singles easy to spot because the cell shows a single note.
What should I do when there are no naked singles?
Look for hidden singles next. If neither technique works, move on to candidate patterns such as naked pairs and pointing pairs.
Does the naked single technique work in Marble Sudoku?
Yes. Marble Sudoku uses nine marble colors instead of the numbers 1 to 9, and the rules are the same. A naked single is an empty hole where only one color is still possible.
PRACTICE IN THE GAME
Find naked singles in a real puzzle
Every Marble Sudoku puzzle uses the same marble board you just played on. Look for the hole that is missing just one color and drop it in.
Next techniqueHidden singles
