You have filled in every naked single and hidden single you can find, the grid is still half empty, and nothing seems to move. This is the moment almost every medium or hard Sudoku reaches, and in most cases the way forward is a technique called Locked Candidates. It comes in two mirror-image forms, usually called Pointing and Claiming (also known as Box-Line Reduction). Once you see the idea behind them, you will start spotting them in nearly every puzzle.
Notation used in this article
Rows are numbered 1 to 9 from top to bottom and columns 1 to 9 from left to right. A cell is written as rXcY: r2c7 means row 2, column 7. The nine 3×3 boxes are numbered 1 to 9 from left to right and top to bottom, so box 1 is the top-left box (rows 1–3, columns 1–3), box 5 is the centre, and box 9 is the bottom-right. A candidate is a digit that could still legally go in an empty cell; many players write candidates as small pencil marks.
The core idea: where a box meets a line
Every box overlaps every row and column that passes through it in exactly three cells. Box 1 and row 2, for example, share r2c1, r2c2 and r2c3. Locked Candidates is nothing more than careful reasoning about this three-cell overlap:
- Each box must contain every digit exactly once.
- Each row and each column must also contain every digit exactly once.
- So if a digit is forced into the overlap from one side, it cannot appear anywhere else on the other side.
The two directions differ only in which unit does the forcing.
| Technique | What you observe | What you remove |
|---|---|---|
| Pointing (box → line) | Inside one box, all candidates for a digit lie in a single row or column | That digit from the rest of that row or column, outside the box |
| Claiming / Box-Line Reduction (line → box) | Inside one row or column, all candidates for a digit lie in a single box | That digit from the rest of that box, outside the row or column |
Pointing: the box points along a line
Suppose we are looking for the digit 5 in box 1. The situation is:
- r1c1, r1c2 and r1c3 are already filled with 3, 8 and 1.
- r2c2 already holds a 9.
- Row 3 already contains a 5, in r3c8, so no cell of row 3 can take another 5.
| Box 1 | c1 | c2 | c3 |
|---|---|---|---|
| r1 | 3 | 8 | 1 |
| r2 | 5? | 9 | 5? |
| r3 | no 5 | no 5 | no 5 |
Row 1 of the box is full and row 3 is blocked, so the 5 of box 1 has only two possible homes: r2c1 or r2c3. We do not know which one yet, but we know for certain that box 1's 5 sits in row 2. Row 2 can only hold one 5, so 5 can be removed as a candidate from every other cell of row 2: r2c4, r2c5, r2c6, r2c7, r2c8 and r2c9. (In this particular grid, box 3 already has its 5 in r3c8, so the cells that really lose a candidate are the empty ones among r2c4–r2c6 in box 2.)
That elimination often triggers something else. Box 2 now cannot take its 5 in row 2, and row 3 already has a 5, so box 2's 5 must be in row 1. If only one cell of box 2's top row is still empty and free of a 5 conflict, you have just found a hidden single.
Pointing works the same way with columns: if every candidate for 4 inside box 7 lies in column 2, then 4 can be erased from column 2 in boxes 1 and 4.
Claiming: the line claims a box
Now turn the logic around. Look at row 4 and the digit 7:
- r4c3, r4c4, r4c6, r4c7 and r4c9 are already filled (with 2, 6, 4, 9 and 1).
- Column 5 already has a 7 in r8c5, so r4c5 cannot be 7.
- Column 8 already has a 7 in r1c8, so r4c8 cannot be 7.
The only cells left for 7 in row 4 are r4c1 and r4c2, and both lie in box 4. Row 4 must contain a 7, so that 7 will be inside box 4. Box 4 can only hold one 7, therefore 7 can be removed from all other cells of box 4: r5c1, r5c2, r5c3, r6c1, r6c2 and r6c3. The row has "claimed" the digit for its part of the box.
Claiming with columns is identical: if the only places for 2 in column 9 are r7c9 and r9c9 (both in box 9), then 2 can be erased from the rest of box 9, in columns 7 and 8.
Pointing or Claiming? A simple test
Both forms remove candidates from the overlap's "outside", but from different units. Ask yourself: which unit am I looking inside?
- Looking inside a box and the candidates form a straight line → Pointing → clean the line.
- Looking inside a line and the candidates fit in one box → Claiming → clean the box.
Eliminations never happen inside the three overlap cells themselves; the candidates there stay exactly as they are.
How to scan for Locked Candidates
- Get your candidates right first. The technique is only as reliable as your pencil marks. A missing or extra candidate leads to a wrong elimination.
- Work digit by digit. Pick one digit, say 6, and look at all nine boxes. For each box where 6 is not yet placed, ask whether its candidates fall in one row or one column. Boxes with two or three candidates for that digit are the most likely to point.
- Then scan the lines. With the same digit, go through the rows and columns. Whenever a line has two or three candidates for it, check whether they all sit in one box.
- Re-check for singles after every elimination. Locked Candidates usually opens up a naked or hidden single nearby, and it is much faster to collect those before looking for the next pattern.
Without pencil marks you can still find pointing pairs by eye: when you are cross-hatching a box for a digit and end up with two possible cells in the same row, mentally block that row for the digit in the neighbouring boxes.
Common mistakes
- Ignoring one candidate. All candidates of the digit in the box (or line) must lie in the overlap. If even one sits outside, there is no locked pattern.
- Removing from the wrong unit. In Pointing you clean the line, not the box; in Claiming you clean the box, not the line. Mixing them up removes valid candidates and breaks the puzzle.
- Erasing inside the overlap. The cells that form the pattern keep their candidates.
- Calling a single a pattern. If only one cell remains, it is simply a hidden single: place the digit.
- Working with outdated marks. After placing a digit, update the candidates in its row, column and box before looking for new patterns.
Why it is the first step after singles
Singles only tell you where a digit goes. Locked Candidates is the simplest technique that tells you where a digit cannot go without pinning down its exact cell. It only involves one digit and one box-line intersection, so it is the natural next step in difficulty. Rating systems reflect this: in the Sudoku Explainer scale, Pointing is rated 2.6 and Claiming 2.8, just below Naked Pairs at 3.0. If you are curious how a puzzle's difficulty is measured, our SE Rating calculator shows which techniques a puzzle needs. In practice, many puzzles labelled medium or hard can be solved with nothing more than singles plus Locked Candidates.
Practice makes it automatic
The first few times, you will find Locked Candidates by deliberate scanning. After a dozen puzzles you will start noticing two pencil marks lined up in a box and seeing the elimination at a glance. Try a medium or hard grid in our online Sudoku, take on today's daily Sudoku, and whenever you get stuck after singles, go digit by digit and look for a box that points or a line that claims.