Choose Super Classes By Refactoring

When I design or code a system, I create concrete classes that are used by my code (i.e. get instantiated). Any superclasses that are not instantiated (often these are called abstract classes) I arrive at by refactoring - in other words, I create them to remove duplication. Therefor I don't care if a circle is an ellipse, or if a square is a rectangle, or vice versa - I let my refactoring tell me.