Thus, in X' theory, the symbol X (or Y, Z, etc.) is a variable standing for the set of category names Adjectives (A), Noun (N), Verb (V), Preposition (P), etc. [...]. Words of the category Noun, Adjective, Verb, etc., will thus have the category symbol N°, A°, V°, respectively (or, more simply, N, A, V).
- Selkirk (2004), a pag.278