Sudoku Techniques: Boxes and Lines (Pointing Pairs)
When the candidates in a box line up in a row or column, you can eliminate that number from the rest of the line.
Pointing pair
Suppose that within a 3×3 box, the cells where a number N can go are all in the same row (or the same column). So that box's N must lie in that row, and you can remove N from the cells of that row in the other boxes.
The reverse pattern (box-line reduction)
Suppose that within a row, the cells where N can go all lie in a single box. So that row's N must lie in that box, and you can remove N from the cells in the other rows of the same box.
How to spot them
Pick one number and look at where it can go in each box. If those cells line up in a straight line, you can use this technique.
More articles: Sudoku Rules and How to Get Started / The Two Basic Sudoku Techniques: Singles (One Number per Cell, One Cell per Number) / How to Use Notes (Candidates) in Sudoku / Sudoku Techniques: Pairs (Naked Pair and Hidden Pair) / Sudoku Techniques: X-Wing / Sudoku Techniques: Trial and Error (for Demon, Hell and Abyss) / How Kids Can Play Sudoku (6×6)
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