Hidden single
A hidden single is a digit that has only one possible cell left in a row, column or box, even though that cell may show other candidates. Because the unit must contain the digit, it goes there. The Bare Sudoku hint reports it as "Only place for 5 in this row", column or box.
When to use it
Whenever a digit is well advanced: once a digit appears five or six times, the remaining boxes are very constrained, and a hidden single is often the next move. Hidden singles are the main technique of Easy and Medium puzzles and remain the workhorse at every level.
How to spot it
Scan one digit at a time. Take a digit, find a box that does not contain it, and cross out every cell of that box that shares a row or column with a placed copy of the digit. If one cell survives, you have found a hidden single. The same scan works on rows and columns. In the game, tap a placed digit: every copy of it is highlighted, so the free cells stand out.
Example
| c1 | c2 | c3 | c4 | c5 | c6 | c7 | c8 | c9 | |
|---|---|---|---|---|---|---|---|---|---|
| r1 | 678 | 4789 | 2478 | 4678 | 4678 | 3 | 269 | 5 | 1 |
| r2 | 1367 | 13479 | 1247 | 5 | 1467 | 1467 | 8 | 2349 | 23469 |
| r3 | 13568 | 13458 | 148 | 9 | 2 | 146 | 7 | 34 | 346 |
| r4 | 2 | 1578 | 178 | 378 | 15789 | 179 | 4 | 6 | 359 |
| r5 | 4 | 57 | 9 | 2367 | 567 | 267 | 1 | 23 | 8 |
| r6 | 158 | 6 | 3 | 248 | 14589 | 1249 | 259 | 29 | 7 |
| r7 | 17 | 147 | 5 | 2467 | 3 | 8 | 269 | 12479 | 2469 |
| r8 | 1378 | 13478 | 6 | 247 | 479 | 5 | 239 | 1234789 | 2349 |
| r9 | 9 | 2 | 478 | 1 | 467 | 467 | 356 | 3478 | 3456 |
Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.
- Box 8 does not contain 9 yet, so one of its empty cells must hold it. The empty cells are r7c4, r8c4, r8c5, r9c5, r9c6.
- In every one of them except r8c5, 9 is blocked by a 9 already placed in the same row or column, so 9 is not a candidate there.
- That leaves r8c5 as the only place for 9 in box 8. Write 9 there, even though r8c5 may still show other candidates.
Common mistakes
- Placing the digit in a cell that is only probably the right one: check every other empty cell of the unit, not just the obvious ones.
- Missing a hidden single because the cell shows several candidates. The other candidates do not matter; what matters is that the digit has no other place in the unit.
- Scanning only boxes. Rows and columns produce hidden singles just as often, especially near the end.
In Bare Sudoku
Needed from level: Easy
In the game, the hint says: Only place for 9 in this box
Frequently asked questions
What is a hidden single in Sudoku?
A digit that can go in only one cell of a row, column or box. The unit must contain the digit, so the cell is decided even if it still has other candidates.
What is the difference between a hidden single and a naked single?
A naked single is about a cell: one candidate left. A hidden single is about a digit: one cell left in a unit. Both place a digit with certainty.
Which is easier to find?
Without notes, hidden singles are usually easier because you can scan the board for one digit at a time. With complete notes, naked singles are the easiest, since a cell with a single candidate stands out.
How does the hint describe a hidden single?
As "Only place for" the digit in this row, this column or this box, with the cell selected. Tap the hint again to place it.
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