Locked Candidates in Sudoku: Pointing and Claiming

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Published April 14, 2026 · Updated October 14, 2026

Puzzle 38 in our sample of 200 from the Hard level starts with 26 digits. Twenty singles later (nine naked singles and eleven hidden singles) it stops: 35 squares open and no single anywhere. Now look at column 9. Its only places for a 4 are R5C9 and R6C9 (rows 5 and 6), its only places for a 6 are the same two squares, and both sit in box 6, the middle box on the right. Whichever takes the 4, box 6 has its 4; the same goes for the 6. So no other square in box 6 can hold either digit, R4C7 is left with only 9, and naked singles do the rest.

That is a locked candidate of the kind called claiming; its mirror image, pointing, runs from the box to the line. Between them they are the first step past singles our puzzles ask for: 23 of 200 Hard puzzles need one or the other, as do 67 of 200 Expert puzzles and 461 of the first 500 in our Evil bank. Easy and Medium puzzles never need them.

What are locked candidates in sudoku?

A box shares a strip of three squares with each row and each column that crosses it. If a digit’s only places in one of the two houses lie inside that strip, the digit is locked there, and the other house cannot have it anywhere outside the strip. The house you start from gives the two kinds:

• Type 1, pointing: within a box, the digit’s places all fall in one row or column, so the digit goes from the rest of that line. Our pointing pairs guide covers it in depth.
• Type 2, claiming: a row’s or column’s places for a digit all lie in one box, so the digit goes from the rest of that box. See our box/line reduction guide.

Puzzle 38’s move is type 2 twice over, on the 4s of column 9 and on its 6s.

Those are the marks the digits alone allow, so our Sudoku Helper shows the same position. Load it there, and Next Hint needs two presses: one to write in the marks, then one that offers a pointing pair on the 4s of box 2 and lists six techniques available, both kinds of locked candidate at the top.

Type 1 and type 2: what is the difference between pointing and claiming?

Only the direction. Pointing asks where a digit can go inside a box; claiming asks where it can go along a row or column. Both can sit on the same digit at once, and puzzle 38 has one of each among its 4s.

Moves: locked-candidate moves our Sudoku Helper played across 200 Hard puzzles, always taking the simplest move. It tries pointing first, which is most of why claiming looks rare. With one square instead of two or three, the pattern would be a hidden single.

How do you find locked candidates on a full board?

Look for both kinds in one pass. The generator behind our puzzles does exactly that when it checks a Hard puzzle: for each house and digit, if the digit’s places all fall inside a second house, the digit leaves the rest of that second house. By hand it is quickest one digit at a time, across three boxes side by side (a band) or one above another (a stack):

1. Take a digit and a band. In each of its three boxes, note which rows still hold the digit. For the 5s in rows 7 to 9 of puzzle 38: box 7 has its 5 already, box 8 has 5s only in row 9, box 9 in rows 8 and 9.
2. A box with the digit in one row is pointing: the digit leaves that row in the other two boxes.
3. A row with the digit in one box is claiming: the digit leaves that box’s other two rows.
4. Then the stacks, by columns, and the next digit.

Start with digits already placed once in the band. Only two rows and two boxes are then open for the digit, so a single gap settles it. Most locked candidates were found there: 39 of the 59 moves on our Hard puzzles, and 165 of the 254 on Expert.

When is a pointing pair also a box/line reduction?

Often. In puzzle 38, rows 7 to 9 already have one 5, at R7C3 in box 7, so rows 8 and 9 must find theirs in boxes 8 and 9, one each.

Box 8’s 5s point along row 9; row 8’s 5s claim box 9. Both arguments strike the 5 in R9C7 and nothing else. The helper finds it both ways: choose Box/Line Reduction from its menu and it shows the claim on row 8, and once its first hint is applied, the next is the pointing move on box 8.

That is no quirk of this puzzle. Once a digit is placed in one box of a band, the other two rows and two boxes must pair off: if a box can use only one of the rows, the other row can use only the other box, and the two readings strike the same marks. All 39 such moves on our Hard puzzles had a twin of the other kind, and all 165 on Expert. Where the band did not yet hold the digit, twins were rare: none of 20 on Hard, 6 of 89 on Expert.

Pointing or claiming: which should you look for first?

Our helper tries pointing first, and its counts follow: 22 of our Hard puzzles needed a pointing move, only 3 a claim. That reflects the order more than the puzzles. At the 59 moments our Hard puzzles ran out of singles, a claim was available 57 times and a pointing move 55 times; on Expert, 251 and 223 times out of 277.

Puzzle 38 shows what a fixed order costs. The helper plays three pointing moves first, on the 4s of box 2 and the 5s and 9s of box 8, and none leaves a single. The two claims in column 9 open the board, and they are all the puzzle needs: without the pointing moves, R4C7 is still left with 9 and naked singles still finish. So scan for both kinds together, band by band, and take the first you find.

How often do sudoku puzzles need locked candidates?

For each of our levels we took 200 puzzles and solved them with the Sudoku Helper’s eighteen techniques, simplest move first, noting every puzzle where a locked candidate was ever the move chosen.

Puzzles on which each kind was ever the simplest move, with the helper trying pointing before claiming. “Either kind” counts a puzzle once even when it needed both. Evil puzzles are filed by score, so the first 500 are tougher than the rest. Measured October 2026.

Puzzles that need one tend to need several: the 23 Hard puzzles took 59 locked-candidate moves between them, up to seven on one puzzle, and only 8 got by with one. On Expert the most was nine. Other sites set their levels differently, so a Medium elsewhere may need locked candidates; ours never does.

Stuck after a locked candidate? What to try next

Look for another. One locked candidate often opens nothing by itself: the move after one was another locked candidate 30 times out of 59 on our Hard puzzles, and 153 times out of 254 on Expert. Check the squares that just lost a mark first, then rescan the band or stack the move touched.

When none is left, our Expert level only ever needs a naked pair. Every one of the first 500 Evil puzzles needs more; the next step is most often a hidden pair (176 of the 500), with fish such as the X-Wing and wings such as the XY-Wing later. Our guide to sudoku techniques carries that ladder all the way up to chains.