When only one candidate remains in a cell, that digit must go there.
After eliminating all digits already present in a cell's row, column, and box, if only one candidate remains, that digit must be placed in the cell. This is the most fundamental and frequently used technique.
Example: If R5C3 sees 1,2,4,5,7 in its row, 3,8 in its column, and 6 in its box, only 9 remains as a candidate, so R5C3 = 9.
| 2 | 6 | 7 | 9 | 5 | 1 | 8 | 3 | 4 |
| 1 | 3 | 4 | 2 | 6 | 8 | 5 | 7 | 9 |
| 5 | 8 | 9 | 3 | 4 | 7 | 1 | 2 | 6 |
| 3 | 4 | 1 | 7 | 2 | 6 | 9 | 8 | 5 |
| 9 | 7 | 8 | 4 | 3 | 5 | 6 | 1 | 2 |
| 6 | 2 | 5 | 18 | 8 | 9 | 3 | 4 | 7 |
4 | 5 | 3 | 6 | 1 | 234 | 7 | 49 | 8 |
47 | 1 | 6 | 8 | 9 | 24 | 24 | 5 | 3 |
| 8 | 9 | 2 | 5 | 7 | 34 | 4 | 469 | 1 |
This 9×9 puzzle is solver-verified to require this technique on its solution path.