Locked candidates

Medium

Also called pointing and claiming. If, within a 3×3 box, a letter can only go in cells that share one row (or column), then that letter must be in the box on that line — so it can be ruled out of the rest of that row/column outside the box. The reverse (a letter in a row/column confined to one box) clears it from the rest of the box.

Try it on a puzzle →