List candidates, then eliminate
For each empty cell, rule out digits already present in its row, its column or its 3×3 box. If only one candidate remains, it is a single. This simple deduction often opens another part of the grid.
Work through one blocked-looking cell
In the example, row 4 is missing 5 and 7. In row 4, column 2, both are possible from the row alone. But column 2 already contains 7, and the cell’s 3×3 box contains 7 as well. So 7 is ruled out and 5 is the only candidate. Apply the same checks to other blanks.
If no candidate disappears
If your notes do not reveal a sure move, recheck them and move to another row, column or box. You can also look for a hidden single: a digit that has only one possible place within a row, column or box. Enter a number only when you can explain why other choices are excluded.
Narrow the candidates in row 4, column 2
Row 4 is missing 5 and 7. Check the column and box, then choose what belongs in the blank.
| Row / Column | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 5 | 3 | 4 | 6 | 7 | 8 | 9 | 1 | 2 |
| 2 | 6 | 7 | 2 | 1 | 9 | 5 | 3 | 4 | 8 |
| 3 | 1 | 9 | 8 | 3 | 4 | 2 | 5 | 6 | 7 |
| 4 | 8 | — | 9 | — | 6 | 1 | 4 | 2 | 3 |
| 5 | 4 | 2 | 6 | 8 | 5 | 3 | 7 | 9 | 1 |
| 6 | 7 | 1 | 3 | 9 | 2 | 4 | 8 | 5 | 6 |
| 7 | 9 | 6 | 1 | 5 | 3 | 7 | 2 | 8 | 4 |
| 8 | 2 | 8 | 7 | 4 | 1 | 9 | 6 | 3 | 5 |
| 9 | 3 | 4 | 5 | 2 | 8 | 6 | 1 | 7 | 9 |
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