Locked Candidates Explained: A Powerful Sudoku Strategy
Sudoku a Day Blog
If you have moved beyond basic elimination techniques like naked singles and hidden singles, the Locked Candidates strategy is your next essential tool. This technique helps you eliminate candidates from specific cells when you know a number must stay within a particular row, column, or box.
What Are Locked Candidates?
Locked Candidates (sometimes called "Pointing Pairs" or "Box/Line Reduction") occurs when a candidate number in a box is confined to a single row or column. Because that number must stay in that row or column within that box, it cannot appear in the same row or column outside the box.
Type 1: Box-Row Locked Candidates
Look at a 3×3 box. If a candidate number appears in only one row within that box, and that row extends outside the box, you can eliminate that candidate from the rest of that row in other boxes.
For example: In a box, the number 7 appears only in cells of the middle row. Since the middle row extends to two other boxes, the 7 cannot appear in those other boxes' cells in that same row. Eliminate 7 as a candidate from those cells.
Type 2: Box-Column Locked Candidates
Similarly, if a candidate number in a box appears only in one column, eliminate that candidate from the rest of that column outside the box.
When to Use This Strategy
Locked Candidates is particularly useful when:
- You have filled in most of the easy numbers
- Candidate grids are getting crowded
- You are solving hard Sudoku puzzles and need to make progress
Practice Makes Perfect
The best way to learn Locked Candidates is to practice. Start with hard Sudoku puzzles and look for boxes where a single candidate is confined to one row or column. With practice, you will spot these patterns automatically.
Ready to practice? Play today's daily Sudoku and try applying this technique.
Related Strategies
Once you master Locked Candidates, explore these advanced techniques:
- Naked Pairs - Eliminate candidates when two cells in a unit have the same two candidates
- Hidden Singles - Find numbers that can only go in one cell
- Expert Sudoku - Tackle the most challenging puzzles