How it works
Pointing pairs (and pointing triples) are the workhorses of hard sudoku. They are the first technique that uses the box rule to constrain rows and columns rather than the other way around.
The pattern is straightforward. In a single 3x3 box, find a digit whose two or three candidate cells all sit on the same row. The digit must go in that box somewhere, and within that box it can only go on that row. Therefore the digit goes in that row, and you can eliminate it from the rest of the row outside the box.
The same logic works for columns. Box-confined candidates that align vertically eliminate the digit from the rest of the column.
When to look for it
Whenever pencil marks are complete. Walk each box and check if any digit's candidates line up on a single row or column.
Tips for spotting the pattern
- Scan each box one digit at a time. The pattern is usually obvious once pencil marks are clean.
- Pointing pairs unlock most hard puzzles. If you are stuck, this is the technique to try first.
- The reciprocal pattern is box-line reduction; learn both to cover the same shape from both angles.
Common mistakes
- Eliminating inside the box. The eliminations apply to the rest of the row or column outside the box.
- Mistaking a hidden single for a pointing pair. If the digit has only one candidate cell in the box, it is a hidden single, not a pointing pair.
- Forgetting that the candidate cells must all be on the same row or column. Even one stray candidate breaks the pattern.