Locked Candidates

Level 4 · Yellow Belt (Orange Stripe)

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

Starting position

Look for Locked Candidates.

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

1

Scan row 3 for 2

Check digit 2 in row 3.

Sudoku grid illustrating step 1: Scan row 3 for 2 — Check digit 2 in row 3.

2

2 is blocked from r3c4 by the column

Digit 2 cannot go in r3c4 because r5c4 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 2: 2 is blocked from r3c4 by the column — Digit 2 cannot go in r3c4 because r5c4 is already in this column. Remove the entry or choose a different digit.

3

r3c5 is already filled

r3c5 is already filled with 9.

Sudoku grid illustrating step 3: r3c5 is already filled — r3c5 is already filled with 9.

4

2 is blocked from r3c6 by the column

Digit 2 cannot go in r3c6 because r7c6 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 4: 2 is blocked from r3c6 by the column — Digit 2 cannot go in r3c6 because r7c6 is already in this column. Remove the entry or choose a different digit.

5

2 is blocked from r3c7 by the column

Digit 2 cannot go in r3c7 because r2c7 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 5: 2 is blocked from r3c7 by the column — Digit 2 cannot go in r3c7 because r2c7 is already in this column. Remove the entry or choose a different digit.

6

r3c8 is already filled

r3c8 is already filled with 8.

Sudoku grid illustrating step 6: r3c8 is already filled — r3c8 is already filled with 8.

7

2 is blocked from r3c9 by column and box

Digit 2 cannot go in r3c9 because r6c9 is in this column and r2c7 is in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 7: 2 is blocked from r3c9 by column and box — Digit 2 cannot go in r3c9 because r6c9 is in this column and r2c7 is in this box. Remove the entry or choose a different digit.

8

2 is confined to box 1

Within this row, 2 appears only in box 1 (a single box).

Sudoku grid illustrating step 8: 2 is confined to box 1 — Within this row, 2 appears only in box 1 (a single box).

9

Row 3 must place 2 in box 1

Because every other row position for 2 lies outside box 1, row 3 must place 2 inside that box.

Sudoku grid illustrating step 9: Row 3 must place 2 in box 1 — Because every other row position for 2 lies outside box 1, row 3 must place 2 inside that box.

10

Other cells in box 1 can't keep 2

Any other cell in box 1 outside row 3 now sees that forced box placement and cannot keep 2.

Sudoku grid illustrating step 10: Other cells in box 1 can't keep 2 — Any other cell in box 1 outside row 3 now sees that forced box placement and cannot keep 2.

11

Remove 2 from the rest of box 1

Remove 2 from the other cells in box 1 outside row 3.

Sudoku grid illustrating step 11: Remove 2 from the rest of box 1 — Remove 2 from the other cells in box 1 outside row 3.

12

Result

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.