Locked Candidates
Sometimes a digit, inside one box, can only sit on one row or one column. You don't know the cell yet, but you know the line's copy of that digit is in this box, and that is enough to clear the rest of the line.
Ninesmith Sudoku ·
In short
- Pointing: in a box, a digit fits only on one row or column. Strike it from that line outside the box.
- Claiming: in a row or column, a digit fits only inside one box. Strike it from the rest of that box.
- Other names: pointing pairs, pointing triples, box/line reduction.
An example
This board is the start of the puzzle on the home page. The two cells left in box 5 are a pointing pair: they point along row 5.
How to spot it
- Scan as you would for a hidden single. When a digit has two or three places left in a box, check whether they share a row or column.
- For claiming, do the same along a row or column: are the digit's places all inside one box?
- With notes, look at one digit across a box: if its notes form a line, that's it.
Why it works
Row 5 needs a 9 somewhere, and so does box 5. Box 5's 9 can only be on row 5, so it is row 5's 9 as well. No other cell of row 5 can take one.
Questions
What are pointing pairs in Sudoku?
Two cells in a box that hold the only places for a digit and lie on the same row or column. They point along that line: the digit comes out of the rest of it.
What is box/line reduction?
The claiming direction: a digit that, in a row or column, fits only inside one box. It comes out of the rest of that box.
Next: X-Wing · all techniques
Drill it in the Pattern Trainer
Ninesmith Sudoku is a calm Sudoku app for iPhone. Its Pattern Trainer drills locked candidates in sessions of 30 to 90 seconds, and when you're stuck the Smith names the technique and shows where to look before anything goes in.
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