- Locked candidates occur when a digit in a box is confined to one row or column
- Pointing: digit locked in a box → eliminate from rest of that row/column outside the box
- Claiming: digit locked in a row/column within a box → eliminate from rest of the box
- This technique is the single most important upgrade from Easy to Medium/Hard solving
- It requires no pencil marks when scanning — just visual identification of digit locations in a box
The Two Types of Locked Candidates
Type 1: Pointing (Box → Line)
When all the possible positions for a digit in a box lie within the same row or column, that digit must go in one of those cells — which means it cannot go anywhere else in that row or column. The digit is "pointing" out of the box into the line.
Example: In the top-left box, digit 4 can only go in row 2 (positions at columns 1 or 3). Therefore, digit 4 cannot appear anywhere else in row 2 across the entire grid — eliminate it from all other row-2 cells outside the top-left box.
Type 2: Claiming (Line → Box)
When all the possible positions for a digit in a row or column lie within the same box, that digit cannot go anywhere else in that box. The row or column "claims" the digit for one specific box.
Example: Digit 7 in row 5 can only go in the center box (columns 4, 5, or 6). Therefore, digit 7 cannot appear elsewhere in the center box — eliminate it from all center-box cells not in row 5.
Why This Technique Is So Useful
Locked candidates eliminations are often free — you can spot them during a box scan without even having pencil marks filled in. They frequently unlock the first stall in a Medium puzzle and create the chain of singles that clears the grid. After Naked Singles and Hidden Singles, locked candidates is the most commonly needed technique in everyday solving.
Finding Locked Candidates Efficiently
- For each box, scan digit by digit
- For each digit, mark which rows and columns the candidates fall in
- If all candidates fall in one row or one column → Pointing pattern found
- After any placement, re-check affected boxes for new pointing patterns
Practice locked candidates in a live Medium or Hard puzzle. They typically appear 3–5 times per Medium puzzle and are often the only technique needed to break a stall. The full techniques library has worked examples with annotated grids.