Looking for a puzzle with Box/Line Reduction…
Box/Line Reduction
Solve one puzzle and let the hint show the technique at work. Unlimited hints, mistakes do not end the run.
Looking for a puzzle with Box/Line Reduction…
Within a row or column, a digit can only appear inside one box, so it is removed from the rest of that box.
Take a row and a digit missing from it. Mark every cell in the row where that digit could still go. If all of them fall inside a single box, the row will place the digit inside that box.
The box therefore has its copy of the digit spoken for, and the digit can be erased from every cell of the box that is not on this row. The same works for columns.
A pointing pair starts in a box and eliminates along a line. Box/line reduction starts on a line and eliminates inside a box. The two are often called locked candidates type 1 and type 2, and most solvers find the second harder simply because scanning a row for a digit is less habitual than scanning a box.
If you have already filled candidates, the tell is visual: all the marks for one digit in a row sit in the same third of it.
The row must contain the digit somewhere, and every available somewhere lies in this box. Whichever cell wins, it is in the box — so the box is done with that digit, and its other cells can drop the candidate.
The eliminations land inside a box, which is the densest unit on the board, so they frequently create a hidden single immediately. On Hard and Expert puzzles this is one of the two or three moves that keep the grid moving.