Smell Parallel Inheritance Hierarchies

Every time you make a subclass of one class, you also have to make a subclass of another. You can recognize this smell because the prefixes of the class names in one hierarchy are the same as the prefixes in another hierarchy.

• Creating subclass in one hierarchy requires subclass in another
• Class name prefixes match across hierarchies
• Two hierarchies grow in parallel

Martin Fowler — Refactoring: Improving the Design of Existing Code, 1st Edition (1999)