Locked Candidates

Level 3 · Yellow Belt

Strategy:

Locked Candidates

Requires:

Learn after:

Empty Rectangle Sue de Coq X-Wing

Overview

Locked Candidates occurs when candidates in a block are restricted to a single row or column. This allows elimination of that candidate from the rest of that row or column outside the block.

How It Works

Type 1 (Pointing): When a candidate in a block exists only in one row/column, eliminate it from that row/column outside the block.
Type 2 (Claiming): When a candidate in a row/column exists only within one block, eliminate it from other cells in that block.

Example

Look for Locked Candidates.

Sudoku grid showing the initial puzzle state for Look for Locked Candidates.

1

Locked Candidates

Check digit 2 in column 1.

Sudoku grid illustrating step 1: Locked Candidates — Check digit 2 in column 1.

2

Locked Candidates

r1c1 is already filled with 6.

Sudoku grid illustrating step 2: Locked Candidates — r1c1 is already filled with 6.

3

Locked Candidates

r2c1 is already filled with 4.

Sudoku grid illustrating step 3: Locked Candidates — r2c1 is already filled with 4.

4

Locked Candidates

r3c1 is already filled with 8.

Sudoku grid illustrating step 4: Locked Candidates — r3c1 is already filled with 8.

5

Locked Candidates

Digit 2 cannot go in r7c1 because r7c5 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 5: Locked Candidates — Digit 2 cannot go in r7c1 because r7c5 is already in this row. Remove the entry or choose a different digit.

6

Locked Candidates

r8c1 is already filled with 5.

Sudoku grid illustrating step 6: Locked Candidates — r8c1 is already filled with 5.

7

Locked Candidates

r9c1 is already filled with 7.

Sudoku grid illustrating step 7: Locked Candidates — r9c1 is already filled with 7.

8

Locked Candidates

Within this column, 2 appears only in box 4 (a single box).

Sudoku grid illustrating step 8: Locked Candidates — Within this column, 2 appears only in box 4 (a single box).

9

Locked Candidates

Because every other column position for 2 lies outside box 4, column 1 must place 2 inside that box.

Sudoku grid illustrating step 9: Locked Candidates — Because every other column position for 2 lies outside box 4, column 1 must place 2 inside that box.

10

Locked Candidates

Any other cell in box 4 outside column 1 now sees that forced box placement and cannot keep 2.

Sudoku grid illustrating step 10: Locked Candidates — Any other cell in box 4 outside column 1 now sees that forced box placement and cannot keep 2.

11

Locked Candidates

Remove 2 from the other cells in box 4 outside column 1.

Sudoku grid illustrating step 11: Locked Candidates — Remove 2 from the other cells in box 4 outside column 1.

12

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

Scan each block looking for candidates aligned in a single row or column.
This technique is very common and should be checked regularly.
Locked Candidates often unlock more complex patterns.