Locked Candidates

What Is It?

Locked Candidates is the first elimination technique most solvers learn after singles. Unlike singles (which place digits), Locked Candidates removes candidates from cells, narrowing down possibilities so that other techniques can finish the job.

There are two sub-types. Pointing occurs when a candidate within a box is confined to a single row or column, so you can eliminate that candidate from the rest of that row or column outside the box. Claiming (also called Box/Line Reduction) is the reverse: when a candidate in a row or column is confined to a single box, you eliminate it from other cells in that box.

This technique appears regularly in Medium puzzles and is essential for progressing beyond Easy difficulty. Once you're comfortable with Locked Candidates, the door opens to pairs, triples, and all the intermediate techniques.

How It Works

Pointing: Look at a single box and pick a candidate digit. If that digit only appears as a candidate in cells that share the same row (or column), those cells "point" along that line. Since the digit must go in one of those cells, it cannot appear anywhere else in that row (or column) outside the box. Remove it from those external cells.

For example, in box 1, suppose digit 5 only appears as a candidate in R1C2 and R1C3. Both cells are in row 1. Since 5 must go in one of these two cells, 5 cannot be in any other cell of row 1, so eliminate 5 from R1C4 through R1C9.

Claiming: Now look at a row (or column) and pick a candidate digit. If that digit only appears in cells that all fall within one box, the row "claims" the digit for that box. Since the digit must appear in one of those cells, it cannot appear in any other cell of that box. Remove it from the other cells in the box.

For example, in row 4, suppose digit 3 only appears as a candidate in R4C1 and R4C3. Both cells are in box 4. Since 3 must go in one of these two cells within box 4, eliminate 3 from all other cells in box 4 (like R5C1, R5C2, R6C3, etc.).

Worked Example

Example 1: Pointing (Box to Line)

Look at box 6 (middle-right) and ask: where can digit 1 go? Scanning the box, most cells are already filled. Only two cells have 1 as a candidate: R5C8 and R6C8.

Both candidate cells sit in column 8. Since 1 must go in one of them within box 6, no other cell in column 8 can contain 1. The digit is "locked" to this box-line intersection.

Remove 1 from R7C8 and R9C8. After this elimination, R7C8 is reduced to a single candidate (3), creating a Naked Single.

Example 2: Claiming (Line to Box)

Now scan column 4 for digit 2. Only two cells in column 4 have 2 as a candidate: R4C4 and R5C4. Both cells fall within box 5 (center).

Column 4 "claims" digit 2 for box 5. Since 2 must occupy one of these two cells, no other cell in box 5 can hold 2.

Remove 2 from R4C6, R5C5, and R6C6. After eliminating, R6C6 drops to a single candidate (8), creating a Naked Single. Place it and continue.

Key Points

• Pointing: candidates locked to one row/column within a box. Eliminate from the rest of that row/column.
• Claiming: candidates locked to one box within a row/column. Eliminate from the rest of that box.
• Both sub-types follow the same logic: if a digit is confined to one intersection of a box and a line, it must go in that intersection.
• This is often the first technique needed to break past the point where singles alone get stuck. If you're stuck on a Medium puzzle, look for Locked Candidates.
• Check every digit in every box systematically. With practice, you'll spot the alignments quickly by noticing when candidates form a straight line within a box.

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All Sudoku Techniques

Explore all 28 solving techniques in our complete technique guide.