Problem is from historical conventions of what a square and rectangle are. A square is not really a thing, its a description of a thing. In this case, proport

Problem is from historical conventions of what a square and rectangle are. A square is not really a thing, its a description of a thing. In this case, proportions of the sides of a shape. So when you translate this into object oriented language, you are subclassing a concept with one of its potential variations. It would be like making a class called MusicalNote and then a subclass named MusicalNoteC, even though MusicalNote already has it covered. So "square" shouldn't be a subclass because its just a proportion of the data within "rectangle", so more appriopriately, 'square' would be a method of Rectangle since it deals with internal data.