Techniques › Hidden Single
Hidden Single
A digit with only one home left in a house, even though that cell has other candidates.
A Hidden Single is a digit that can only go in one cell of a row, column or box — even though the cell itself still has several candidates. The digit is hidden among them.
This is the single most productive technique in sudoku and the one that carries almost every easy puzzle from start to finish. It is also the one most people under-use, because it requires scanning by digit rather than by cell: pick a number, look at a box, and ask where it can still go.
A worked example
Taken from a real puzzle in our bank, at the point where this was the cheapest move available. Pattern candidates are marked in amber; anything the step eliminates is struck through in red.
| 7 | 6 | 17 | 8 | 24 | 9 | 5 | 13 | 24 |
| 5 | 8 | 249 | 3 | 24 | 1 | 249 | 7 | 6 |
| 29 | 13 | 249 | 6 | 7 | 5 | 1 | 249 | 8 |
| 37 | 2 | 8 | 179 | 5 | 6 | 149 | 49 | 47 |
| 4 | 1 | 157 | 79 | 38 | 378 | 239 | 6 | 257 |
| 6 | 9 | 57 | 2 | 13 | 4 | 8 | 1 | 57 |
| 12 | 57 | 26 | 17 | 1368 | 378 | 47 | 458 | 9 |
| 19 | 4 | 69 | 5 | 16 | 278 | 27 | 28 | 3 |
| 8 | 57 | 3 | 4 | 9 | 27 | 6 | 25 | 1 |
- NudgeA digit has only one home left in box 1. Find it.
- WhereHidden Single in box 1: r3c2
- Do itr3c2 = 3
How to spot it
Take one digit at a time. For each box, cross off the rows and columns that already contain it. If exactly one cell survives, that is your placement. Boxes first — they are the easiest to scan and yield the most.
Where you will meet it
Hidden Single sits at tier 0 of our ladder, which means it first becomes necessary at Very Easy. Puzzles below that band never require it, though it may still appear as a shortcut.
Read next
- Full House A house with one empty cell left. The first thing anyone learns.
- Naked Single A cell with only one candidate left.
- Pointing A digit confined to one line inside a box clears that digit from the rest of the line.