Locked candidates: pointing and claiming

Locked candidates are the first technique that goes beyond singles. They rely on one observation: when a digit’s possible positions in a box all sit in one row or column, the digit is “locked” to that line. The same idea works the other way round. Once you can see it, you can remove candidates that looked safe before.

What is a locked candidate?

A locked candidate is a digit whose possible cells in one unit all lie inside a second unit. The digit must go somewhere in the first unit, so it must go in one of those shared cells. That means it can be removed from every other cell of the second unit. There are two forms, called pointing and claiming.

How it works

Pointing: from a box to a line

In pointing, the box is the starting unit. Suppose every possible cell for a digit inside a box sits in one row of that box. The digit must go in that row, inside the box. So the same digit can be removed from the rest of that row, outside the box.

In box 1, rows 1 and 3 are already full (1, 2, 3 and 4, 6, 7), so the 5 that box 1 still needs can only go in r2c1, r2c2 or r2c3. Those cells are marked x. Because the 5 in box 1 must be in row 2, no cell in row 2 outside box 1 can be 5. Those cells are marked -.

Claiming: from a line to a box

In claiming, the row or column is the starting unit. Suppose every possible cell for a digit in a row sits inside one box. The digit must go in that box, within that row. So the digit can be removed from the other cells of the same box, outside the row.

Row 5 already holds 1, 8, 3, 4, 5 and 6, so its only empty cells are r5c4, r5c5 and r5c6 (marked x) – and all three lie in box 5. The 7 that row 5 still needs must therefore sit in box 5, inside row 5. That means no other cell of box 5 can be a 7: remove 7 from r4c4, r4c5, r4c6, r6c4, r6c5 and r6c6 (marked -). Nothing else on this board rules out those six cells – claiming is the only reason they lose the 7.

How to spot it

• Pick a box and a digit. List the cells in the box where the digit is still possible. Do this for each missing digit.
• Check the shape. If all those cells share a row or column, it is pointing. Remove the digit from that line outside the box.
• Reverse the check. Pick a row or column and list where a missing digit is possible. If all those cells lie inside one box, it is claiming. Remove the digit from the rest of that box.
• Use pencil marks. Locked candidates are hard to see without notes. Hush’s notes help you check each digit’s cells quickly.

A worked example

Here is a pointing example in column form. The picture shows box 9, the bottom-right box, and the digit 2.

Start with box 9. Its cells in columns 7 and 8 are already filled (1, 3, 4 and 5, 6, 7). The 2 that box 9 still needs can only go in column 9, marked x in rows 7 to 9. Since the 2 in box 9 must sit in column 9, it cannot sit in column 9 in rows 1 to 6. Those cells are marked -.

Notice that nothing else on the board rules out a 2 in r1c9 to r6c9 – pointing is the only reason those six cells lose it. In a real puzzle the other cells of the box are often blocked by 2s in nearby rows and columns rather than by filled cells, but the logic is the same.

Common mistakes

• Removing digits from the wrong place. For pointing, the removal is in the line outside the box. For claiming, it is in the box outside the line. Removing inside the starting unit is wrong.
• Forgetting that the digit must be possible. A locked candidate needs at least one possible cell in the first unit. If the digit is already placed in that unit, there is nothing to do.
• Checking a box that already has the digit. If the digit is already placed in the box, there is nothing to lock. Pick a digit the box still needs.
• Mixing up the two forms. Pointing starts in the box. Claiming starts in the line. Name the direction before you remove anything.

Why locked candidates matter

Locked candidates are useful because they work on puzzles where singles stop. A grid can look completely stuck, with no naked or hidden singles in sight, and still yield to a pointing or claiming move. Each move is small, but several small moves often unlock the rest of the grid.

They also teach an important habit: think about where a digit is allowed to go across two units at once. Pairs, triples and the fish patterns in later guides all build on that habit. If you are working through the Learn hub in order, locked candidates are the bridge between the singles-only techniques and the pair-based ones.

Harder puzzles, including the Expert level, rely on these same ideas in more complex shapes. Expert sudoku explains how Hush rates difficulty by technique rather than by the number of givens, which is why a puzzle with many givens can still be demanding. The app’s home page describes the levels in one place.

Practice locked candidates in Hush

Locked candidates appear in Hush’s Medium level, together with naked and hidden pairs. Medium is unlimited and free, and the daily puzzle is a Medium puzzle from Monday to Thursday, so you can practice every day. Once pointing and claiming feel natural, try naked pairs next, then hidden pairs. To review the basics first, read how to solve sudoku. If you are comparing apps, the Sudoku.com alternative page describes how Hush’s levels differ from that app.