Who owns the interface?

In the early 1990s, we used to think that the physical bond ruled. There were very reputable books that recommended that inheritance hierarchies be placed together in the same physical package. This seemed to make sense because inheritance is such a strong physical bond. But over the last decade, we have learned that the physical strength of inheritance is misleading and that inheritance hierarchies should usually not be packaged together. Rather, clients tend to be packaged with the interfaces they control.

This misalignment of the strength of logical and physical bond is an artifact of statically typed languages like C++ and Java. Dynamically typed languages, like Smalltalk, Python, and Ruby, don't have the misalignment because they don't use inheritance to achieve polymorphic behavior.

Robert C. Martin, "Abstract Server, Adapter, and Bridge", in Agile Software
Development: Principles, Patterns, and Practices, 319.