Naked Single
Sole Candidate
A cell sees 8 different digits in its row, column and box, so only one possibility is left.
Rule
A cell's row + column + box contain 8 different digits → the cell is the 9th.
Pick a cell
Look at cell r4c5. It belongs to row 4, column 5 and box 5 at the same time — the three units it "sees".
The idea
This technique looks from the cell's side. Each cell "sees" 20 others: 8 in its row, 8 in its column and 4 more in its box. Collect the digits in those cells; if there are 8 different ones, the missing digit is the answer.
Once every cell has its candidates written in, naked singles stand out: they are cells with a single candidate. Without pencil marks you have to count, which is why this technique usually comes after the hidden singles run out.
How to spot it
- Favour cells where a busy row crosses a busy column.
- With pencil marks: look for a cell with just one small digit.
Common mistakes
- Counting the same digit twice (say a 4 in both the row and the box) and thinking you already have 8.
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Related techniques
References
The examples on this page are generated from random puzzles, not taken from these sources.