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 6,9 in its box, only one candidate remains and is placed.
| 3 | ||||||||
| 8 | ||||||||
| 6 | ||||||||
| 1 | ||||||||
| 9 | 2 | 6 | 3 | 4 | 5 | 1 | 8 | 7 |
| 2 | ||||||||
| 5 | ||||||||
| 7 | ||||||||
| 9 |
R1C1=5, R1C2=3, R1C4=6, R1C6=9, R1C8=8 → Missing digits in row 1: 1,2,4,7.
If column of R1C3 has 2,7 and its box has 4 → R1C3 = 1.
This 9×9 puzzle is solver-verified to require this technique on its solution path.