Sudoku locked candidates usually appear at the exact moment a grid starts to look annoying. The easy singles are gone. Your notes have multiplied. A number seems possible in too many places, and the whole board begins to feel like a fogged window.

Then one box gives away a secret.

The number may not be solved yet, but it is trapped. It can only sit on one row inside a 3x3 box, or one column inside that box. That little trap lets you erase the same candidate from the rest of the row or column.

That is the charm of Sudoku locked candidates. You do not place a number. You clean the board so the next number has room to show itself.

If you want a relaxed place to test the habit, open a fresh puzzle on [Easy Daily Puzzle](https://easydailypuzzle.com/) and try one pass where you look only for locked notes before guessing anything.

Sudoku locked candidates start with one trapped digit

Classic Sudoku asks for the digits 1 through 9 in every row, column, and 3x3 box. [Nikoli's Sudoku page](https://www.nikoli.co.jp/en/puzzles/sudoku/) describes that clean rule as the heart of the puzzle. Locked candidates sit right on top of it.

Here is the plain version.

Imagine the digit 7 can go in only two cells inside the upper-left 3x3 box. Both cells are also in the same row. That means the 7 for that box must be somewhere in that row segment. Because that row can contain only one 7, every other 7 note outside the box but still on that row can be erased.

Nothing dramatic happened. No final answer landed in the grid. Still, the board improved.

This is why Sudoku locked candidates feel different from hidden singles or naked singles. A single gives you a number. A locked candidate gives you permission to delete a possibility.

That deletion matters.

A messy Sudoku grid is often not hard because it has no logic. It feels hard because too many pencil marks are competing for your attention. The locked candidate technique works like a small broom. It sweeps away candidates that the box-row relationship has already disproved.

Pointing and claiming without the jargon headache

Strategy guides usually divide Sudoku locked candidates into two related forms. [SudokuWiki calls this family intersection removal](https://www.sudokuwiki.org/Intersection_Removal), because the move depends on how a row or column intersects a 3x3 box. [HoDoKu explains the same idea](https://hodoku.sourceforge.net/en/tech_intersections.php) under intersections, with pointing and claiming examples.

The names are useful, but only after the idea feels clear.

Pointing starts inside a box. A digit is limited to one row or one column inside that box, so it points outward. You can remove that digit from the rest of the same row or column outside the box.

Claiming starts with a row or column. A digit in that row can only appear inside one box, so the line claims the box. You can remove that digit from the other cells in the same box.

That is all.

Box points to line. Line claims box.

If that sounds a little too neat, good. Sudoku names can make small moves sound like expensive machinery. In practice, you are only asking one question: is this digit trapped in the overlap between a box and a line?

If yes, erase the places outside the trap.

If no, leave the notes alone.

The best time to look for Sudoku locked candidates

Do not start a puzzle by hunting locked candidates immediately. That is how a simple Sudoku turns into paperwork.

Start with obvious scans first. Look for rows, columns, and boxes that already have many numbers. Find singles. Fill the clean placements. Only then should the locked-candidate pass begin.

The best trigger is a board with notes that feel crowded but not hopeless.

For example, you may notice that digit 4 appears in three cells across a row, but two of those cells sit inside the same box. Or you may notice that a box has only two possible locations for digit 9, and both locations fall in one column. Those are the moments worth checking.

Use one digit at a time.

Pick 1 and scan the boxes. Then 2. Then 3. Do not jump around because the notes look interesting. Locked candidates reward a calm rhythm. When you scan one digit across all nine boxes, patterns become easier to see.

[Sudoku.com explains pointing pairs](https://sudoku.com/sudoku-rules/pointing-pairs/) as a note-based technique where a candidate appears twice in one block and the two notes sit on the same row or column. That is a good beginner doorway because it keeps the eye focused on one block first.

Once that doorway is comfortable, claiming feels less strange. You simply reverse the direction.

What to erase, and what not to erase

The dangerous part of Sudoku locked candidates is not the pattern. It is the eraser.

People get excited when they find a locked candidate and start cleaning too much. The move only removes candidates that the overlap proves impossible. It does not solve every cell that shares the same digit. It does not let you guess the final position inside the box.

Use this slow check before you erase.

First, identify the digit. Do not say "this box has something." Say "the 6 is trapped."

Second, identify the trap. Is the digit trapped inside one row of a box, one column of a box, or one box along a row or column?

Third, name the erase zone. For pointing, erase outside the box along the same row or column. For claiming, erase inside the box but outside the line.

Fourth, leave the trapped cells alone. Those cells are still candidates unless another rule solves them.

This last part matters most. A locked candidate is often a cleaning move, not a placement move. If you turn it into a guess, the technique loses its value.

The board should feel lighter after the move, not finished.

A small example you can picture

Picture the middle-left box. You are checking digit 5.

Inside that box, 5 can only go in two cells, and both cells sit in row 5. Since every 3x3 box needs one 5, the box's 5 must be somewhere in row 5.

Now look across row 5 outside that box. If you see any other 5 notes, erase them. The box has already claimed the row segment for its own 5.

You have not solved the 5 inside the box. It could still be either of the two cells. But the rest of row 5 just got cleaner.

That cleaning may uncover something else. Maybe a different cell now has only one candidate. Maybe another box now has a hidden single. Maybe the next locked candidate becomes easier to see because the row is less crowded.

This is how intermediate Sudoku often moves. Not with one giant breakthrough, but with a few careful removals that make the next small fact visible.

Sudoku locked candidates are useful because they slow down guessing

Most casual players do not get stuck because they lack advanced techniques. They get stuck because guessing starts to feel faster than proving.

Sudoku locked candidates give you something better to do before guessing.

They ask for proof that is visible on the board. Is the candidate trapped? Does the row or column force it into one box? Does the box force it into one line? If the answer is yes, remove the disproved notes. If the answer is no, keep scanning.

That is a satisfying middle step between easy singles and advanced patterns like X-Wing.

It also makes note-taking less ugly. A page full of candidates can scare off a relaxed player. But if those notes keep getting cleaned by real logic, the grid stays readable. You begin to trust the process because the board is not just collecting possibilities. It is losing bad ones.

For more puzzle practice, browse [Easy Daily Puzzle's Sudoku search](https://easydailypuzzle.com/?s=sudoku). Try one puzzle where your goal is not speed. Your goal is to find one locked candidate and erase only what it proves.

One clean erasure is enough for the technique to click.

The next time your notes start crowding the grid, do not panic and do not guess. Pick one digit. Scan the boxes. Look for a number trapped in a row or column.

The grid may not hand you the answer yet. It may do something better: remove the wrong paths.