Well in fact, many people would say that surely a concept such as 'height' is NOT the same when talking about different sorts of things. For example: (height (make-instance 'elm-tree)) ;in FEET (height (font-ref *default-hardcopy-font* #\a)) ;in INCHES (height (make-instance 'window)) ;in PIXELS Looks more like an issue of units than an issue of datatype to me.