Box-Line Reduction
All of a digit's candidates in a row or column fall inside one box, so it can be eliminated from the rest of that box.
How it works
- 1Row 1 is drawn in full: digit 6 survives only at A1 and C1, and both of those sit in the top-left box.
- 2Nothing earlier fires here — no cell is down to one candidate, no digit has a single home in the row or the box, and no digit inside the box is penned into one row or column.
- 3Row 1's 6 therefore has to land in this box, so the box's other rows give theirs up — cross 6 off A2 and B3.
- Cells that establish the pattern
- Where the conclusion lands
- Candidate ruled out