Pointing line
When a digit's only homes inside a box all share a row or column.
The pointing line (also known as locked candidates, type 1) is the first move that erases candidates instead of placing a digit. If every cell in a box where a digit could still land sits on the same row, then the digit's eventual placement is somewhere on that segment — and so the digit can be wiped from the rest of that row outside the box. Same logic applies for columns.
It doesn't fill a square. But it tightens the candidate grid, and that's usually enough to set off a chain of singles a step or two later.
When the move applies
Pointing lines appear naturally as boxes fill in. When a digit has two or three remaining homes inside a box, glance at the row and column numbers — alignment on a single line is the trigger.
The procedure
- Choose a box and a digit.
- Mark every cell in the box where the digit is still a candidate.
- If those cells all lie on the same row, scrub the digit from the rest of that row.
- If they all lie on the same column, scrub the digit from the rest of that column.