Your post is an excelent example of why inheirtance relationships are so hard to get right (even for this seemingly trivial example). From a Eucldian point of view, a square ISA rectangle. However, in OOP, a subclass is supposed to extend the parent's behavior, so it makes more sense to say a rectangle ISA square. Two groups thus create two opposite designs.

So who is right? It depends on your application. As you say, if this is a program for geometry, it's probably better to say a square ISA rectangle. If it's a CAD program, then it's probably better to say a rectangle ISA square.