>It's only "perfectly expected" if you know about the implementation details of Square, which may not even have existed until after a lot of Rectangle code was

>It's only "perfectly expected" if you know about the implementation details of Square, which may not even have existed until after a lot of Rectangle code was already in place. I don't quite agree. You don't need to know implementation details of Square (e.g. does SetHeight set the height and call SetWidth, or does it just call SetSide? Or does it, as well as SetWidth, set some new property called side?), you just need to know how a Square differs from a Rectangle (width == height). Because Squares are a subset of Rectangles, any Rectangle code should also work with Squares. Even without Square, prevents you instantiating a Rectangle and always calling SetWidth(x) and SetHeight(x) one after the other.