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Sudoku techniques · Hard

Box/Line Reduction in Sudoku

Box/Line Reduction runs a Pointing Pair backwards. If a number's only remaining spots in a row or column all fall inside one box, then that box must spend the number on that line, so the number can be erased from every other cell of the box.

Also known as: Locked Candidates Type 2, Claiming.

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Why it works

If a number on a line only fits inside one box, that candidate is removed from the rest of that box outside the line.

How to spot it

Scan a line, not a box. Take one number and mark where it can still go along a full row or column.

  • If every surviving cell falls inside a single box, the rest of that box loses the candidate.
  • Pointing and Box/Line Reduction are the same observation read from two directions, so check both whenever you find one.
  • The eliminations land in cells you were not looking at, which is what makes this feel like progress out of nowhere.

Builds on

Pointing Pair

Worked example

These are the app's own lesson boards. Step through them the same way a hint does.

PatternAnswerContextBlockerElimination
Step 1 of 2
Box/Line Reduction: the pattern as the app draws it during a hint.
  1. 1

    In this row, every cell where 7 can still go lies inside the same box.

  2. 2

    So we can remove the candidate 7 wherever it appears in that box outside that row.

Where it fits

The game looks for the easiest technique that applies before it offers you a harder one. Box/Line Reduction sits at position 7 of 37 on that ladder, which is why puzzles that need it are graded Hard.

Learn before thisPointing TripleLearn after thisNaked Pair

Other Hard techniques

Pointing PairPointing TripleNaked PairHidden PairNaked TripleHidden Triple