Pointing line

Pointing line (sometimes called 'locked candidates type 1') is the first elimination technique. If a digit's only candidate positions inside a box all fall on a single row or column, then that digit must come from one of those cells — and can be removed from the rest of that row or column outside the box.

It doesn't directly place a digit. Instead, it shrinks the candidate field, often opening up naked or hidden singles elsewhere. It's the bridge between 'find the next placement' solving and 'reduce the puzzle' solving.

When to look for it

Look for pointing lines whenever a box has 2–3 cells where a particular digit can still go, all aligned on the same row or column. This is especially common after a few placements have eaten up other cells.

How to apply it

1. Pick a box and a digit.
2. Mark every cell in the box where that digit is still a candidate.
3. If those cells all lie on the same row, eliminate the digit from the rest of that row.
4. If they all lie on the same column, eliminate it from the rest of that column.

Example