What it is
Box-line reduction is the mirror of pointing pairs. If every candidate for a digit in a row or column falls inside a single box, the box must supply that digit to the line — so the digit can be removed from the box’s other cells.
How to spot it
Work line by line. For each missing digit of the row or column, mark its possible cells; when they cluster inside one box, sweep the remainder of that box.
Interactive example
Use Next to walk through the deduction on the board.
Step by step
- Where can 6 go in the bottom row? Five cells are filled, and the 6 higher up blocks the sixth column.
- Every remaining 6-candidate in the row falls inside the bottom-right box.
- The row must take its 6 from those cells, so 6 can be removed from the rest of that box — box-line reduction.
Common mistake: Mixing up the direction: here the line restricts the box (eliminate in the box). In pointing pairs the box restricts the line (eliminate along the line).
Put it into practice
Examples on this page are validated with the Play Sudoku Daily puzzle engine.