In linguistics, X-bar theory is a model of phrase structure and a theory of syntactic category formation that proposes a universal schema for how phrases are organized. It suggests that all phrases share a common underlying structure, regardless of their specific category (noun phrase, verb phrase, etc.). This structure, known as the X-bar schema, is based on the idea that every phrase (XP, X phrase) has a head, which determines the type (syntactic category) of the phrase (X).
The theory was first proposed by Noam Chomsky in 1970 reformulating the ideas of Zellig Harris (1951), and further developed by Ray Jackendoff (1974, 1977a, 1977b), along the lines of the theory of generative grammar put forth in the 1950s by Chomsky. It aimed to simplify and generalize the rules of grammar, addressing limitations of earlier phrase structure models. X-bar theory was an important step forward because it simplified the description of sentence structure. Earlier approaches needed many phrase structure rules, which went against the idea of a simple, underlying system for language. X-bar theory offered a more elegant and economical solution, aligned with the thesis of generative grammar.
X-bar theory was incorporated into both transformational and nontransformational theories of syntax, including government and binding theory (GB), generalized phrase structure grammar (GPSG), lexical-functional grammar (LFG), and head-driven phrase structure grammar (HPSG). Although recent work in the minimalist program has largely abandoned X-bar schema in favor of bare phrase structure approaches, the theory's central assumptions are still valid in different forms and terms in many theories of minimalist syntax.
Background
The X-bar theory was developed to resolve the issues that phrase structure rules (PSR) under the Standard Theory had.
The PSR approach has the following four main issues.
- It assumes exocentric structures such as "S → NP Aux VP". This is contrary to the fact that phrases have heads in all circumstances. John talked to the man in person involves the PSR of "VP → V (PP) (PP)". This indicates that it is necessary to posit new PSRs every time when an undefined structure is observed in E-language, which amounts to adding an indiscriminate number of grammatical rules to Universal Grammar. This poses serious issues from the perspectives of the Plato's problem and the poverty of the stimulus.
The X-bar theory is a theory that attempts to resolve these issues by assuming the mold or template phrasal structure of "XP".
X-bar schema
Basic principles
The "X" in the X-bar theory is equivalent to a variable in mathematics: It can be substituted by syntactic categories such as N, V, A, and P. These categories are lexemes and not phrases: The "X-bar" is a grammatical unit larger than X, thus than a lexeme, and the X-double-bar (=XP) outsizes the X(-single)-bar. X-double-bar categories are equal to phrasal categories such as NP, VP, AP, and PP. This structure is called the X-bar schema.
thumb|none|300px|Figure 1
As in Figure 1, the phrasal category XP is notated by an X with a double overbar. The category I includes auxiliary verbs such as will and can, clitics such as -s of the third person singular present and -ed of the past tense. This is consistent with the headedness principle, which requires that a phrase have a head, because a sentence (or a clause) necessarily involves an element that determines the inflection of a verb.
Assuming that S constitutes an IP, the structure of the sentence John studies linguistics at the university, for example, can be illustrated as in Figure 10.
thumb|none|450px|Figure 12
In this derivation, the I-to-C movement is an instance of subject-auxiliary inversion (SAI), or more generally, head movement.
Other phrasal structures
- VP-internal subject hypothesis: A hypothesis on the inner structure of VP proposed by researchers such as Yuki Kuroda (1988), Dominique Sportiche (1988), Fukui and Speas (1986) and Kitagawa (1986). It assumes that the sentential subject is base-generated in Spec-VP, not in Spec-IP.
- DP Hypothesis: A hypothesis proposed by Abney (1987), according to whom noun phrases are not NPs but DPs headed by the functional category D.
- VP shell: An analysis put forth by Larson (1988), which assumes two-layered structures of VP. Later in Chomsky (1995a, 1995b 2001), according to whom small clauses are PredPs headed by the functional category Pred.
- Bare Phrase Structure (BPS): A replacement of the X-bar theory put forth by Chomsky (1995a,
This sentence is ambiguous between the reading I saw a man, using binoculars, in which with binoculars modifies the VP, and the reading I saw a man who had binoculars, in which the PP modifies the NP.), and Revised Extended Standard Theory (Chomsky, 1981