A class-expression is either a class name (some of which have predefined interpretations), an enumerated-class, a slot-constraint, a conjunction of class expressions, written , a disjunction of class expressions, written or a negated class expression, written .
The class names top, thing and bottom have pre-defined interpretations: top and thing are interpreted as the most general class (written ), while bottom is interpreted as the inconsistent class (written ). Note that top and bottom can just be considered as abbreviations for the class expressions and respectively (for some arbitrary class ).
An enumerated-class consists of a list of individual names (introduced by the one-of key word) written .