Locked Candidates: Pointing & Claiming

Locked Candidates is an elimination technique that exploits the overlap between a 3×3 box and a row or column. When a digit's candidates within a box all sit on the same line — or a line's candidates all sit in the same box — you can eliminate that digit from the rest of that line or box. There are two flavors: Pointing and Claiming.

Prerequisites

Naked Single — be comfortable filling forced cells before applying line-box eliminations.

Pointing (box → line)

Look inside a 3×3 box. If a digit's candidates are all in one row (or column), that digit cannot appear elsewhere in that row outside the box. Remove it from the other six cells in that row.

Box 1 (rows 1–3, cols 1–3) — digit 7:

Row 1: ·  ·  ·  │ ·  7? ·  │ ·  ·  ·
Row 2: ·  7? 7? │ ·  ·  ·  │ ·  ·  ·   ← 7 locked to row 2 in box 1
Row 3: ·  ·  ·  │ ·  ·  ·  │ ·  ·  ·

→ Remove 7 from cols 4–9 in row 2 (outside box 1)

Because 7 must go somewhere in box 1, and row 2 is the only row available, 7 cannot appear in row 2 anywhere outside box 1.

Claiming (line → box)

Look along a row or column. If a digit's candidates are all within one 3×3 box, that digit cannot appear elsewhere in that box. Remove it from the other cells in that box that are not on your line.

Row 5 — digit 3:

Row 5: ·  ·  ·  │ 3? 3? ·  │ ·  ·  ·
                    ↑── both in box 5 (rows 4–6, cols 4–6)

→ Remove 3 from rows 4 and 6 within cols 4–6 (rest of box 5)

Because 3 in row 5 is confined to box 5, it must land there. The other cells in box 5 (rows 4 and 6) can therefore be cleared of the digit 3.

Why it works

The Sudoku constraint is that every digit appears exactly once per row, column, and box. When a digit's candidates within a box are confined to one line, that line's allocation to this box is fixed — the digit will end up in that line, inside that box. Nothing outside the box on that line can also hold the digit. The claiming argument is the exact reverse: confine the digit to a box-line intersection and clear the rest of the box.

Common mistakes

  • "Mostly" isn't "all". Locked Candidates only fires when every candidate for a digit within a box (or line) is in the intersection. If one stray candidate falls outside, the pattern doesn't apply.
  • Wrong direction. Pointing eliminates from the line outside the box. Claiming eliminates from the box outside the line. Swapping these is the most common error.
  • Forgetting to re-check. After an elimination, other Locked Candidate patterns may now be visible. Repeat the scan after each placement.

Frequently asked questions

What is a Locked Candidate in Sudoku?

A Locked Candidate occurs when all positions for a digit in a box are on one line (Pointing), or all positions for a digit on a line are in one box (Claiming). In both cases you can eliminate the digit from the cells in that line or box that fall outside the intersection.

What is the difference between Pointing and Claiming?

Pointing starts from a box and eliminates along a line. Claiming starts from a line and eliminates within a box. The pattern is the same constraint seen from two directions — candidates locked to a box-line intersection.

How do I find Locked Candidates?

For Pointing: scan each box digit-by-digit. Ask "are all candidates for this digit on one row or column?" For Claiming: scan each row and column digit-by-digit. Ask "are all candidates for this digit inside one box?" Both scans take seconds once practised.

Is Locked Candidates harder than Hidden Singles?

Slightly. Hidden Singles require looking at one unit (row, column, or box) at a time. Locked Candidates require looking at the overlap between two units simultaneously — a box and a line. Most solvers master it after a few Medium puzzles.

What comes after Locked Candidates?

The next step up is X-Wing — a pattern that spans two rows and two columns to eliminate candidates at scale. After X-Wing, Swordfish extends the logic to three rows and three columns.

Next technique: X-Wing · All Techniques