Learn how Locked Candidates (Pointing and Claiming / Box-Line Reduction) work in Sudoku with visual 9x9 diagrams to eliminate candidates across rows, columns, and boxes.
Locked Candidates (also known as Intersection Removal) is the bridge between beginner scanning and intermediate candidate elimination. Even when you cannot immediately place a digit into a single cell, you can often prove that a digit within a 3×3 box is restricted to a single row or column—allowing you to erase that digit from other cells along the line.
When all candidate positions for a specific digit (e.g., 5) inside a 3×3 box lie on a single row (or column), one of those cells MUST contain 5. Therefore, candidate 5 can be eliminated from all other cells in that same row (or column) outside the box.
| 5? | 5? | → | 5✕ | → | 5✕ | → | 5✕ | . |
| 2 | 8 | 9 | . | . | . | . | . | . |
| 4 | 7 | . | . | . | . | . | . | . |
Looking at a single row (or column): if all possible spots for digit 5 in that row fall inside a single 3×3 box, then that row "claims" digit 5 for that box. You can eliminate candidate 5 from all other cells inside that 3×3 box that are not on that row.
