Box/Line Reduction

Level: first needed in Expert puzzles. In one line: In a row or column, a number fits only inside one box, so it is removed from the rest of that box.

Box/Line Reduction, also called Claiming, is the mirror image of a pointing pair. Look at a row (or column) and a number. If every possible place for that number in the row falls inside the same box, then the row "claims" the number for that part of the box.

Because the box's copy of the number must be the row's copy, it cannot be anywhere else in the box. Remove it from the box's other cells.

Example from a real puzzle

This position comes from an Extreme puzzle in Sudoku Arena, at the exact moment the Box/Line Reduction is the next logical move. Cells are named by row and column: R3C4 means row 3, column 4.

In row 4, 9 must be inside box 4, so it is removed from the rest of that box. Result: remove 9 from R5C2.

Play this puzzle and look for the move yourself. It's a practice game, so it doesn't count towards your stats.

How to spot it

1. Pick a row or column and a number that is not yet placed in it.
2. Look at where the number can still go along the line.
3. If all of those cells are inside a single box, the number is locked into that part of the box.
4. Remove the number from the box's cells that are not on that line.

Tips

• Pointing pairs go from a box to a line; box/line reduction goes from a line to a box. Checking both directions finds most "intersection" moves.
• Rows and columns with only two or three gaps left are the easiest places to spot it.

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