Hidden members of Class objects

[Carl Sturtivant via Digitalmars-d-learn]

I notice that a class with no data members has a size of two 
words (at 64 bits). Presumably there's a pointer to a table of 
virtual functions, and one more. Is the Vtable first?

A COM class that inherits from IUnknown and has no data members 
has a size of three words, presumably as before plus something 
that ordinarily is in the Vtable but can't be for a COM object as 
it has its own layout of that.

What is actually in these objects using that space?