Pointing Pairs and Box-Line Reduction are two complementary forms of the locked candidate technique. Both eliminate possibilities by exploiting overlap between a 3×3 box and a row or column - but they point in opposite directions. Together they form one of the most powerful intermediate patterns in Sudoku.
A Pointing Pair (or pointing triple) occurs when all candidates of a digit inside a 3×3 box fall on the same row or column. The candidate must be placed somewhere in that box, so it must be on that row or column - which means the digit can be eliminated from the rest of that row or column outside the box.
Box-Line Reduction is the reverse: when all candidates of a digit inside a row or column fall within a single 3×3 box, the digit must end up inside that box - so it can be eliminated from the other cells in that box outside the row or column.
Both techniques exploit the intersection of a box and a line. Pointing eliminates from the line beyond the box. Box-Line eliminates from the box beyond the line. Together they cover every possible elimination produced by box-line interaction. Skipping either one leaves easy progress on the table.
Most solvers find Pointing Pairs faster because boxes feel like natural visual units. Train yourself to also scan rows and columns for digits restricted to a single box. A simple habit: after finding a Pointing Pair on one digit, immediately check whether that digit forms a Box-Line Reduction elsewhere.
These two techniques resolve the bulk of intermediate puzzles. They unlock cells that pure Naked Singles and Hidden Singles cannot reach, and they often expose a chain of follow-on eliminations. Mastering both is the gateway to confidently solving hard puzzles.
They are mirror images. Both use the box-line intersection but eliminate in opposite directions.
Pointing Pairs - boxes are visually distinct units. Box-Line requires scanning rows and columns systematically.
Yes. Pointing Triples (3 cells in a line within a box) and 3-cell Box-Line Reductions are common. The logic scales naturally.
After Naked and Hidden Singles, before Naked and Hidden Pairs. They unlock progress that pairs and triples often need to reveal themselves.
Yes - any Sudoku variant that keeps the row/column/box structure also keeps locked candidates as a valid technique.
Almost all medium and hard puzzles. Easy puzzles rarely need them, but they are essential from medium upward.
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