Locked Candidates Sudoku Technique Explained

The critical bridge between basic and advanced techniques

Locked Candidates example in Sudoku

What are Locked Candidates?

Locked Candidates occur when a candidate within a box is restricted to a single row or column. This "locks" the candidate to that line, allowing eliminations in the rest of that row or column outside the box.

This technique comes in two forms: Pointing (box to line) and Claiming (line to box), both based on the same logical principle.

The Locked Candidates Principle: When a candidate in a box is confined to one row or column, that candidate must appear somewhere in that intersection. It can be eliminated from the rest of the row/column outside the box.

Two Types of Locked Candidates

Type 1: Pointing

A candidate in a box appears only in one row or column

Direction: Box → Line

Elimination: Remove from rest of that row/column

Type 2: Claiming

A candidate in a row/column appears only in one box

Direction: Line → Box

Elimination: Remove from rest of that box

Type 1: Pointing (Box to Line)

How to Find Pointing Candidates:

  1. Choose a box: Select any 3×3 box
  2. Pick a candidate: Choose a number that hasn't been placed in that box
  3. Check its positions: Where can this candidate appear within the box?
  4. Look for alignment: Are all positions in the same row or column?
  5. Make eliminations: Remove the candidate from that row/column OUTSIDE the box

Pointing Example

In Box 1, the candidate 7 can only appear in cells R1C2 and R1C3 (both in row 1).

Logic: Since 7 must go somewhere in Box 1, and it can only go in row 1, the 7 for Box 1 will be in row 1.

Elimination: Remove 7 from all other cells in row 1 (outside Box 1).

Type 2: Claiming (Line to Box)

How to Find Claiming Candidates:

  1. Choose a row or column: Select any line
  2. Pick a candidate: Choose a number that hasn't been placed in that line
  3. Check its positions: Where can this candidate appear in the line?
  4. Look for box confinement: Are all positions within the same box?
  5. Make eliminations: Remove the candidate from that box OUTSIDE the line

Claiming Example

In Row 5, the candidate 3 can only appear in cells R5C4 and R5C6 (both in Box 5).

Logic: Since 3 must go somewhere in row 5, and it can only go in Box 5, the 3 for row 5 will be in Box 5.

Elimination: Remove 3 from all other cells in Box 5 (outside row 5).

Why Locked Candidates Matter

  • Bridge technique: Connects basic singles to advanced patterns
  • High frequency: Appears in most medium and hard puzzles
  • Powerful eliminations: Often unlocks multiple cells at once
  • Foundation for fish: Understanding locked candidates helps with X-Wing and Swordfish
Pro Tip: Scan boxes systematically for locked candidates. For each box, check if any candidate is confined to a single row or column. This is often faster than checking lines first.

Common Patterns

Pointing Pair

Two cells in a box, same row/column, share a candidate that points outside

Pointing Triple

Three cells in a box, same row/column, share a candidate that points outside

Box/Line Reduction

Another name for Claiming – the line "claims" the candidate for its box intersection

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