A locked candidate is a digit whose remaining possibilities inside a house all fall on the same line - or vice versa. Spotting locked candidates lets you eliminate that digit from cells far outside the originating house.
A locked candidate is a digit whose only homes inside one constraint group all sit on a different shared constraint group. Concretely, in a 3x3 box, if a digit can only go in cells of one row, that digit is locked to that row. The digit must end up there, so it cannot also live in cells of that row outside the box.
The reverse pattern is equally useful: when all candidate cells for a digit in a row (or column) fall inside a single box, the digit is locked to that box and can be eliminated from the box's other rows or columns.
Pointing (also called Pointing Pairs / Triples) goes from box to line: the box restricts the digit to a single row or column, and you eliminate from the rest of that line.
Claiming (also called Box-Line Reduction) goes from line to box: a line restricts the digit to a single box, and you eliminate from the rest of that box. Both are the same logic viewed from opposite directions.
For each digit, scan every box. When the digit's candidates inside the box all sit in one row or column, you have a Pointing pattern. Then sweep rows and columns: when a digit's candidates in a line all sit inside one box, you have a Claiming pattern.
Locked candidates often appear right after you have placed singles for a while. They are the natural follow-up technique. See the strategies index for next steps once locked candidates run out.
Pointing pairs are one specific case of locked candidate. The umbrella term 'locked candidate' covers both Pointing (box -> line) and Claiming / Box-Line Reduction (line -> box) patterns.
Yes. The pattern relies on knowing exactly where a digit can still go inside a house. Without complete pencil marks, you cannot reliably tell when all candidates fall on one shared line.
After naked and hidden singles run out and before tackling pairs. Locked candidates frequently re-open singles and are the cheapest non-trivial technique in your toolkit.
We're tracking the digit 5. Look at the upper-left 3×3 box. Where can 5 fit there? Only in two cells of row 1.
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