PIENSALIA / SUDOKU EDUCATION

Locked Candidates in Sudoku

Locked Candidates connect the candidate positions in a box with one crossing row or column.

Level: Learn how Locked Candidates use box and line intersections to eliminate candidates from rows and columns.

Prerequisites

Know the three Sudoku rules and keep candidates consistent across each row, column, and 3 × 3 box.

How to apply it

1. Scan the relevant units and write only legal candidates.
2. Compare the positions or candidates described by the technique.
3. Make one justified placement or elimination.
4. Update related notes and rescan for new singles.

Worked example

If every possible 5 in a box lies on its top row, other cells on that row outside the box cannot contain 5.

Common mistakes

• Guessing before checking every related unit.
• Leaving stale candidates after a placement.
• Applying a pattern without verifying all of its positions.

When to use it

Use the technique after singles and pairs when candidates cluster along one edge of a box.