When a candidate is confined to one row or column within a box, it reveals powerful eliminations.
Also known as: Pointing Pairs/Triples, Claiming (Box/Line Reduction)
Look for a digit whose candidate cells inside a box all fall on one row or column (Pointing), or whose candidates in a row or column all fall inside one box (Claiming). The digit is locked to that box-line intersection, so clear it from the rest of the other unit: the line for Pointing, the box for Claiming.
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.
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.).
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.
Pointing runs from a box to a line: a digit confined to one row or column inside a box is removed from the rest of that row or column. Claiming runs the other way: a digit confined to one box within a row or column is removed from the rest of that box.
Never in the cells that form the pattern. Those candidate cells sit in the intersection of a box and a line, and the intersection is exactly where the digit must end up, so it stays there. Pointing clears the line outside the box; Claiming clears the box outside the line.
No. Locked Candidates removes candidates rather than placing digits, which is what separates it from Naked and Hidden Singles. The payoff is indirect: after the eliminations a nearby cell is often left with one candidate, and that Naked Single is what you place. Elimination first, placement second.
No, they are the two halves of Locked Candidates. Pointing Pairs and Pointing Triples name the box-to-line form, after how many cells the digit is confined to, while Box/Line Reduction is the usual name for Claiming, the line-to-box form. The underlying logic is shared, but the eliminations land in opposite units.
Because singles eventually run out. Locked Candidates is usually the first technique a solver learns that removes candidates instead of placing digits, and those removals restart the flow of singles. If a Medium puzzle has stalled and no unit yields a single, box-line alignments are the first thing to check.
Work through the digits box by box. For each box, look at where a digit can still go and check whether those cells form a straight line, all in one row or all in one column. That alignment is the whole pattern. Then repeat the sweep across rows and columns to catch Claiming.
Explore all 28 solving techniques in our complete technique guide.