The top of the class hierarchy should be abstract

This idea is very tightly related to Riel's heuristic Avoid concrete base classes. It states that the class at the top of a class hierarchy should be abstract, and if it is not then there may be a design problem. If a subclass B overrides methods in its concrete superclass A, it may indicate that there should be a common abstract superclass to A and B which defines common behaviour; then neither A or B need to redefine any methods. Any example of this is given in Avoid concrete base classes.