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:
For more detailed information on the properties of lexical categories, see the page on parts of speech from the Selected Subfields section (Syntactic Theory) of ELLO.
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)