Finding where a number belongs: hidden singles on a 6×6
A hidden single is a cell that is the only place a particular digit can go within a row, column, or box. Other digits may still look possible in that cell, but the digit has nowhere else to go in that group, so it must go there.
This is the technique behind Sudoku Sama’s daily question: where a given number belongs in a given row. The pages here are reading guides. You play in the app.
What a hidden single is
A hidden single is about one digit and one group. The group is a row, a column, or a 2×3 box. You are not asking which digits fit a cell. You are asking where this digit can still sit inside that group.
Hidden single vs naked single
| Hidden single | Naked single | |
|---|---|---|
| What you notice | The only place for this digit in a group | The only digit for this cell |
A naked single is a cell where only one digit fits. A hidden single is a digit that fits in only one cell of a group. Both are proved placements, found from different directions. A candidate is only a note that a digit is not ruled out yet.
The method: scan one digit across one group
To find where a number belongs in a row, list the row’s empty cells, then rule out each cell whose column or box already contains that number. If only one cell is left, that is where the number goes.
- Pick the group the question names, such as row R, and the digit N.
- List that group’s empty cells.
- For each empty cell, check whether N is already in that cell’s column or its 2×3 box.
- Cross out the blocked cells. If one cell remains, that placement is proved, not guessed.
“Not ruled out yet” is not the same as “proved.” The 6×6 rules make that distinction: a candidate is not an answer until the constraints force it.
Three places a hidden single hides on 6×6
In a row
The row case is the daily shape of the question: where does N go in row R? The worked one-move example proves one of these, with 6 in row 3. This page does not rebuild that grid.
In a 2×3 box
Where does 1 go in the bottom-right box (rows 5–6, columns 4–6)? It goes in R5C5. The grid below is an original teaching example, not today’s Sudoku Sama puzzle and not a puzzle from the Premium archive. It is a static figure, not a game.
| Row / column | |||||
| 1 | 4 | Empty | 2 | Empty | 5 |
| 2 | Empty | 5 | 1 | Empty | Empty |
| 4 | 6 | Empty | Empty | 5 | 1 |
| 3 | 5 | 1 | Empty | Empty | 2 |
| 6 | Empty | Empty | Empty | Empty | Empty |
| 5 | 1 | 4 | Empty | Empty | Empty |
- Name the group. The bottom-right box is rows 5–6 and columns 4–6. None of those six cells is a given 1, and R5C4, R5C5, and R5C6 are empty.
- Column 4 is blocked. R2C4 is already 1, so R5C4 cannot be 1.
- Column 6 is blocked. R3C6 is already 1, so R5C6 cannot be 1.
- Row 6 is blocked. R6C2 is already 1, so R6C4, R6C5, and R6C6 cannot be 1.
Every cell in the box is ruled out except R5C5. That cell’s row and column do not already contain 1, and the box does not either. R5C5 = 1. The placement follows from those givens. You do not have to fill the other empty cells first.
In a column
Turn the same scan sideways. Pick column C and digit N, and list the empty cells in that column. A row that already holds N rules out that row. A 2×3 box that already holds N rules out the cells of the column inside that box. If one cell remains, N belongs there.
Why 2×3 boxes change the scan
A box spans 2 rows and 3 columns, so it blocks 3 cells of a row but only 2 of a column. Scanning a row against boxes and scanning a column against boxes are the same rule with a different reach. Why the boxes are 2×3 covers the shape. This page stays with the scan.
When there’s no hidden single yet
Sometimes every remaining cell in the group is still open. Nothing is proved yet. Fill another forced cell first, or check a different group, then scan again. That is why one key placement can take several deductions. The final answer is still one cell. Not every daily puzzle is a hidden single on the first look.
Practice in the daily puzzle
Sudoku Sama asks one such question each day on a 6×6 grid: where a given number goes in a given row. Sometimes the answer is a hidden single straight away. Sometimes it takes a few deductions first. There is no timer. After you answer, HOW? replays the numbered steps. It is a step replay, not a glossary of technique names. The app is in English for iPhone and iPad, and it requires iOS or iPadOS 26 or later. The how-to guide covers the controls.
Download Sudoku Sama on the App Store
Daily 6×6 · one key placement · no timer · today’s puzzle free
FAQ
What is a hidden single?
It is the only cell in a row, column, or box where a particular digit can go. The digit has to go there, even if that cell still looks open to other digits.
Is a hidden single the same as a naked single?
No. A naked single is the only digit that fits in one cell. A hidden single is the only cell in a group that fits one digit.
Does this work on 9×9?
Yes. The idea is the same on a classic grid. The boxes are 3×3 instead of 2×3. Mini Sudoku vs classic 9×9 compares the two formats.
Can I practise on this website?
No. The guides are here. You play in the app.
Download Sudoku Sama on the App Store
Daily 6×6 · one key placement · no timer · today’s puzzle free
About Sudoku Sama · 6×6 rules · Worked example · How to play · One key placement · Why boxes are 2×3