Short quiz
The following words form a sort of class in English. Why?
new, normal, regular, mad, crazy
(There are actually two separate subclasses, but at the right level of granularity, you can call it a single class. Also, this may not be a completely exhaustive list, though it is as far as the BNC is concerned.)
(Hint: I started looking for this class of words while doing the laundry.)