Technique guide
Locked candidates
Locked candidates use the overlap between one 3×3 box and one row or column. If every candidate for a digit in the box lies on the same line, that digit cannot appear elsewhere on the line.
When to use it
Use this after singles stop appearing and your candidate notes are reliable. The reverse direction—when all candidates in a line fall inside one box—is often called box-line reduction.
A small example
Inside the top-left box, suppose both possible positions for 6 lie in row 2. One of them must eventually be 6, so any 6 notes in row 2 outside that box can be removed.
Step by step
- 1Pick a box and a missing digit with two or three candidates.
- 2Check whether every candidate lies on one row or one column.
- 3Remove that digit only from the rest of that same line, outside the box.
Recommended level: Intermediate.
Put it into practice
Medium boards are classified with box-line interactions among their modeled tools.