Naked Single Sudoku Technique
The first technique anyone learns and the one that finishes most of every grid. A cell has one candidate left, so that candidate is the answer.
Also called: sole candidate, forced digit, last remaining candidate.
What a naked single is
A naked single is a cell whose candidate list has been reduced to exactly one digit. The other eight are already used somewhere in its row, its column, or its 3x3 box, so there is nothing else it could be.
It is called naked because the cell shows you the answer directly: you do not have to reason about any other cell to place it. Every other technique on this site exists only to create naked singles somewhere else in the grid.
How to spot one
- Work cell by cell rather than digit by digit. For an empty cell, run 1 through 9 and cross off anything already in its row, column or box; if one digit survives, place it.
- Prioritise cells in crowded units. A cell in a box that already holds six digits, on a row that holds five, has far fewer survivors than one in an empty corner.
- If you keep pencil marks, a naked single is simply a cell showing one mark, which is why turning candidates on makes easy grids almost mechanical.
Worked example
| Column 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
|---|---|---|---|---|---|---|---|---|---|
| Row 5 | 8 | 24 | 1 | 469 | 7 | 6 | 3 | 5 | 29 |
Solid digits are already placed. Small digits are the candidates still available in that cell.
- Column 6 and box 5 between them already account for 2, 4 and 9, so the sixth cell is left holding only a 6.
- Place the 6. Every other cell in row 5, in column 6, and in box 5 now loses 6 as a candidate.
- Re-scan those units. Removing one digit routinely uncovers the next naked single, which is why easy grids unravel in a chain once the first one falls.
What it eliminates
Placing the digit removes it as a candidate from all twenty peers of that cell — the other eight cells of its row, its column, and its box.
When you need it
Everywhere, at every difficulty. An Easy grid at 46 clues can usually be finished with naked and hidden singles alone; a 26-clue grid still ends this way, it just takes more work to reach the first one.
Difficulty here is set by how many clues are left in the grid: Easy starts from 46, Medium 40 and Hard 34, with Extreme and Insane stripped further still. Fewer clues is what makes the techniques below necessary — the fewer digits you start with, the deeper you have to reason.