Intermediate
Box-line reduction
The mirror move: a digit on a line is trapped inside one box.
Box-line reduction (locked candidates, type 2) is the same idea as pointing line, viewed from the other end of the telescope. Take a row or a column. If every place where a particular digit could still land falls inside one box, then the digit's eventual home is one of those cells — and you can remove it from the rest of that box.
Like pointing line, it's an elimination move. It doesn't write anything new on the board; it just makes the candidate grid honest.
When the move applies
Watch rows and columns that have a digit pinned to just a couple of cells. If those cells happen to share a box, the move fires.
The procedure
- Pick a row or column and a digit.
- Find every cell on that line where the digit is still a candidate.
- If all those cells sit inside one box, remove the digit from the rest of that box.