Box/Line Reduction

Intermediate

When a digit in a row or column is confined to one box, it can be eliminated from the rest of that box.

How It Works

The reverse of Pointing Pairs. When a digit in a row or column can only appear within a single box, it can be eliminated from all other cells in that box (those not in the shared row/column).

Since the digit must be in that row/column within the box, it cannot appear elsewhere in the box.

Example

142583
69
7
39
369
59
569
16
47
7
29
8
49
367
58
567
16
47
67
59
2
169
267
158
156
346
345
456
138
359
27
367
158
156
246
245
456
18
359
27
237
158
157
34
345
457
138
356
26
469
269
469
178
157
157
35
345
28
469
269
46
178
157
157
35
345
28
247
26
47
368
356
568
135
134
26
Key cells of the techniqueCells where elimination is appliedCandidate to be placedEliminated candidate (crossed out)

In Row 4, digit 3 is only a candidate at R4C4, R4C5, R4C6 (all in Box 5) → Eliminate 3 from all other cells in Box 5 (rows 5 and 6).

Practice with a Real Puzzle

This 9×9 puzzle is solver-verified to require this technique on its solution path.

Easy32 givensStrict