An important aspect of a good object-oriented design is strong separation between an abstraction and its implementation. Multiple structural patterns deal specifically with this area. so the next part of the book is dedicated to them. I'll start with the bridge pattern and then dedicate the next chapter to other patterns of this kind.
The bridge pattern introduces a strong separation between an interface and its implementation. By this approach, we define the abstraction part as one inheritance hierarchy (a group of derived classes) and the implementation part as another hierarchy. The main point is that the abstraction and implementation hierarchies have nothing in common. The only connection between them is that the abstraction part is using implementation classes through composition.