Locked Candidates Explained: A Powerful Sudoku Strategy
By Michel ·
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 use the overlap between a box and a row or column. Pointing starts with a digit confined to one line inside a box and removes it farther along that line. Claiming starts with a digit confined to one box along a line and removes it from the other cells of that box.
Type 1: Pointing from a box
Look at a 3×3 box. If all remaining positions for a digit lie in one row, remove that digit from the rest of the row outside the box. The same rule works for a column: remove only the candidates outside the box on that column.
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.
For the full walkthrough, see the Pointing Pairs strategy guide.
Type 2: Claiming from a row or column
Start with a row or column. If all remaining positions for a digit lie inside one box, that line must place the digit there. Remove the digit from the other cells of that box, outside the starting line. For example, if row 4 can place 6 only at R4C1 or R4C3, remove 6 from rows 5 and 6 within box 4. This is claiming, also called box-line reduction.
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 a Sudoku puzzle and the candidate pattern supports an elimination
Practice Makes Perfect
Practise both directions on today's Sudoku puzzle: scan a box for pointing, then a row or column for claiming. Check all remaining positions for the digit before removing a candidate. No particular difficulty label is required.
Ready to practice? Play today's daily Sudoku and try applying this technique.
Related Strategies
Compare these related lessons and practice options:
- Pointing Pairs (Locked Candidates Type 1) — the primary strategy guide for this technique
- Box-Line Reduction (Locked Candidates Type 2) — the complementary technique for eliminating inside the box
- 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
- Daily Sudoku - Choose a difficulty and practise at your own pace
Frequently Asked Questions
What are locked candidates in sudoku?
Locked candidates occur when all remaining positions for a digit in one unit lie in its intersection with another unit. Pointing removes candidates outside the box along a line; claiming removes candidates inside the box but outside the starting line.
What is the difference between locked candidates type 1 and type 2?
Type 1 (Pointing): the candidate in a box is confined to one row or column, so you eliminate it from that row or column outside the box. Type 2 (Box-Line Reduction): the candidate in a row or column only falls in one box, so you eliminate it from the rest of that box.
When should I use the locked candidates technique?
Look for locked candidates when a digit is restricted to a box-line intersection in your puzzle. You can inspect this pattern independently of naked pairs; neither technique is a prerequisite for the other.