Box/line reduction is the pointing pair’s mirror image. This time a row (or column) does the claiming: if every remaining candidate for a digit in that line falls inside one box, the box must place the digit on that line — so the digit can be erased from the box’s other two lines.
To spot one, sweep a row or column for a single digit and note which boxes its candidates fall in. All in one box? Follow the box and strike that digit from its cells that are not on your line.
It works because the line must contain the digit somewhere, and all of its options sit inside that one box. The box’s copy of the digit is therefore committed to the line — no other cell of the box may take it.
On the board
Sweep row 6 for the digit 1. Its only possible homes are the first three cells — R6C1, R6C2, and R6C3 — which all sit inside the left-middle box. Row 6 must get its 1 from that box, so the box’s 1 is committed to row 6. That erases the 1 from the box’s other cells: R4C1, R4C2, R5C1, and R5C2 all lose a pencil mark, tightening the whole corner of the grid.
This is the exact board the app's interactive Box/Line Reduction demo uses — step 22 of its 60-step walkthrough, drawn here with its live pencil marks.
