The X-Bar Model: Lexical Categories
I. Lexical categories
Lexical categories (i.e. verbs, nouns, adjectives, adverbs, and prepositions) have specific properties which distinguish them from functional categories:
(a) Semantic criteria: They carry semantic meaning.
(b) Morphological criteria: They are marked morphologically.
(c) Distributional (syntactic) criteria: They can be characterized by their distribution.
The features that are often taken to constitute the lexical (and phrasal) categories are [+/-noun] and [+/-verb]. Thus, the lexical categories can be decomposed into the following features:
II. Lexical categories and syntactic structure
Lexical categories function as the head of phrases. Thus, a verb phrase (VP) is a constituent whose head is a verb (V). Analogously, a noun phrase (NP) is a constituent whose head is a noun (N); an adjective phrase (AP) is a constituent whose head is an adjective (A); a prepositional phrase (PP) is a constituent whose head is a preposition (P).
A head may require an obligatory XP as a sister. Sisters to X° are called complements. In contrast, optional YPs providing additional information are referred to as adjuncts; they are sisters to X'. (cf. the principles of X-Bar Theory described above)