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Locked Candidates (Pointing & Claiming)
Master locked candidates: when a digit in a box is confined to one row or column, eliminate it elsewhere. Pointing and claiming explained.
Locked candidates are your first real elimination technique — they remove candidates rather than place a digit. There are two mirror-image forms.
Pointing: if every spot for a digit inside a box lies on the same row (or column), that digit must end up on that line inside the box — so you can erase it from the rest of the line outside the box.
Claiming (box-line reduction): if a digit in a row (or column) can only appear within one box, you can erase it from the other cells of that box.
These eliminations unlock medium puzzles and set up naked and hidden singles. The example highlights a box where a digit is locked to one line.
Practise the Locked candidates
The best way to learn a technique is to use it. Play a puzzle at the level where it first appears, or drop a tricky board into the solver to watch it in action.
Frequently asked questions
What is the difference between pointing and claiming?
Pointing goes from a box to a line: a digit confined to one line within a box is removed from the rest of that line. Claiming goes from a line to a box: a digit confined to one box within a line is removed from the rest of that box.
When do I need locked candidates?
Mostly on medium puzzles, when naked and hidden singles stall. They reopen the board so simpler moves resume.
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