Empty Rectangle
When a box pattern combines with an outside strong link.
Explanation
If one candidate inside a box can only appear along one row and one column, it creates an empty rectangle. When an outside conjugate pair aligns with one side of that pattern, the opposite intersection can remove the candidate.
Practice tip
Scan one digit at a time inside each box. Look for candidates forming an L, T, or plus shape, then check matching outside row or column pairs.
Example steps
- Find a box where one candidate is confined to one row and one column.
- Find an outside strong link that touches one side of the box pattern.
- Remove the candidate from the opposite intersection.