Locked Candidates (Pointing and Claiming)

Locked Candidates is the standard intermediate-to-advanced technique for exploiting the overlap between a 3×3 box and a row or column. Every box intersects three rows and three columns, and candidates inside a box can sometimes be confined to just one of those lines — or vice versa. That confinement lets you eliminate candidates outside the overlap.

The technique has two flavours with different names depending on which way you reason:

Pointing: candidates confined within a box are restricted to one row or column, letting you eliminate that digit from the rest of the row or column outside the box.
Claiming (also called Box/Line Reduction): candidates confined to one line inside a single box let you eliminate that digit from the rest of the box outside the line.

Both are really the same observation — a digit with only one possible "lane" out of a region — seen from opposite directions.

Pointing

The rule

If, inside a 3×3 box, a particular digit can only appear on a single row (or a single column), then that digit must be somewhere on that row inside the box. It cannot appear anywhere else on that row outside the box.

Worked example

Consider the top-left 3×3 box. Suppose 5 can only appear in two cells, and both cells happen to be in row 2. Whatever the final placement, the 5 for this box lands somewhere in row 2.

That means no other cell in row 2 (outside the box) can be a 5. We can delete 5 from every candidate list in row 2 for cells in the top-middle and top-right boxes.

Claiming (Box/Line Reduction)

The rule

If a digit in a row (or column) can only appear inside the section of that row that overlaps a single box, then that digit for that row must live in that box. So the digit cannot appear in any other cell of that box outside the row.

Worked example

Look at row 4. Suppose 7 can only appear in two cells of that row, and both happen to be inside the middle-left box. One of those two cells will hold the row's 7. The rest of the middle-left box — the cells in rows 5 and 6 within that box — must therefore not have 7.

That lets you strike 7 from the candidate lists of every cell in the middle-left box that is not in row 4.

Pointing vs. Claiming: Same Idea, Different Anchor

The difference is where you started looking. Pointing starts with a box ("inside this box, where can the digit go?") and looks out to a line. Claiming starts with a line ("inside this row/column, where can the digit go?") and looks into a box.

In practice, experienced solvers don't label the two separately. They just notice that a digit has been squeezed into a single lane, then follow the elimination in whichever direction the evidence points.

How to Find Locked Candidates

Pick a box. Scan the candidate lists of empty cells within the box.
For each digit missing from the box, note the rows and columns in which that digit still has candidates.
If a digit only has candidates in one row of the box, you have a pointing pair or pointing triple. Eliminate that digit from the rest of the row outside the box.
If a digit only has candidates in one column of the box, apply the same logic using the column.
Then flip the view. For each row and column, check whether any missing digit is confined to a single box. If so, apply the claiming elimination.

Triples, Not Just Pairs

Locked candidates don't have to involve only two cells. If a digit can appear in three cells of a box, and all three sit on the same row, the pointing elimination still applies. The same goes for claiming — the "line reduction" can involve two or three cells, as long as they are all inside the same box.

Common Mistakes

Eliminating in the wrong direction. When pointing, eliminate along the row/column outside the box. When claiming, eliminate inside the box outside the row/column. It's easy to go the wrong way when the pattern is new.
Acting on "almost" patterns. If a digit has candidates in two rows of a box, the technique doesn't apply — it only works when all candidates sit on exactly one line.
Forgetting to re-pencil. After a locked-candidate elimination, re-check for naked and hidden singles. Locked-candidate moves frequently create follow-up placements.

Why This Technique Matters

Locked candidates are usually the tipping-point technique for Hard puzzles. Scanning, naked singles, and hidden singles clear the low-hanging fruit; naked and hidden pairs clean up some candidate lists; and then locked candidates are the tool that cuts through the middle of the grid and opens the remaining cells to more placements.

Expert puzzles often need locked candidates several times in a single solve, and the technique often precedes advanced patterns like X-Wing and Swordfish.

Related Techniques

Hidden Pairs — similar reasoning (where can this digit go?) applied within a single unit.
Naked Pairs — the cell-first view that pairs well with the digit-first view used here.
X-Wing — the natural next step, extending the same kind of elimination across two units.

Practice

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Last reviewed on April 23, 2026.