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X-bar theory is a component of linguistic theory which attempts to identify syntactic features common to all languages. It claims that among their phrasal categories, all languages share certain structural similarities, including one known as the "X-bar", which does not appear in traditional phrase structure rules. X-bar theory was first proposed by Chomsky (1970)¹ and further developed by Jackendoff (1977) ².

The letter X is used to signify an arbitrary lexical category; when analyzing a specific utterance, specific categories are assigned. Thus, the X may become an N for noun, a V for verb, an A for adjective, or a P for preposition.

The term X-bar is derived from the notation representing this new structure. Certain structures are represented by X (an X with an overbar). Because this is difficult to typeset, this is often written as X′, using the prime symbol. In English, however, this is still read as "X bar". The notation XP stands for X Phrase, and is equivalent to X-bar-bar (X with a double overbar), written X″, usually read aloud as X double bar.

Contents 1 Core concepts 2 How the rules combine 3 A simple noun phrase 4 A full sentence 5 Substitution test 6 Reduction 7 See also 8 References 9 External links

Core concepts

There are three "syntax assembly" rules which form the basis of X-bar theory. These rules can be expressed in English, as "rewrite" rules (useful for programmers), or visually as parse trees. All three representations are presented below.

1. An X Phrase consists of an optional specifier and an X-bar, in any order:

XP → (specifier), X′

     XP                XP
    /  \      or      /  \
spec    X'           X'  spec

2. One kind of X-bar consists of an X-bar and an adjunct, in either order:

(X′ → X′, adjunct)

Not all XPs contain X′s with adjuncts, so this rewrite rule is "optional".

   X'                   X'
  / \        or        / \ 
X'   adjunct    adjunct   X'

3. Another kind of X-bar consists of an X (the head of the phrase) and any number of complements (possibly zero), in any order:

X′ → X, (complement...)

  X'                                X'
 / \             or                / \  
X   complement           complement   X  

(a head-first and a head-final example showing one complement)

How the rules combine

The following diagram illustrates one way the rules might be combined to form a generic XP structure. Because the rules are recursive, there is an infinite number of possible structures that could be generated, including smaller trees that omit optional parts, structures with multiple complements, and additional layers of XPs and X′s of various types.

    XP
   / \
spec  X'
     / \
    X'  adjunct
   / \
  X   complement
  |
head

Because all of the rules allow combination in any order, the left-right position of the branches at any point may be reversed from what is shown in the example. However, in any given language, usually only one handedness for each rule is observed. The above example maps naturally onto the left-to-right phrase order used in English.

Note that a complement-containing X' may be distinguished from an adjunct-containing X' by the fact that the complement has an X (head) as a sister, whereas an adjunct has X-bar as a sister.

A simple noun phrase

The noun phrase "the cat" might be rendered like this:

     NP
    /  \
  Det  N'
   |   |
  the  N
       |
      cat
 

The word the is a determiner (specifically an article), which at first was believed to be a type of specifier for nouns. The head is the determiner (D) which projects into a determiner phrase (DP or DetP). The word cat is the noun phrase (NP) which acts as the complement of the determiner phrase. More recently, it has been suggested that D is the head of the noun phrase.

Note that branches with empty specifiers, adjuncts, complements, and heads are often omitted, to reduce visual clutter. The DetP and NP above have no adjuncts or complements, so they end up being very linear.

In English, specifiers precede the X-bar that contains the head. Thus, determiners always precede their nouns if they are in the same noun phrase. Other languages use different orders. See word order.

A full sentence

For more complex utterances, different theories of grammar assign X-bar theory elements to phrase types in different ways. Consider the sentence He studies linguistics at the university. A transformational grammar theory might parse this sentence as the following diagram shows:

The "IP" is an inflectional phrase. Its specifier is the noun phrase (NP) which acts as the subject of the sentence. The complement of the IP is the predicate of the sentence, a verb phrase (VP). There is no word in the sentence which explicitly acts as the head of the inflectional phrase, but this slot is usually considered to contain the unspoken "present tense" implied by the tense marker on the verb "studies".

A head-driven phrase structure grammar might parse this sentence differently. In this theory, the sentence is modeled as a verb phrase (VP). The noun phrase (NP) that is the subject of the sentence is located in the specifier of the verb phrase. The predicate parses the same way in both theories.

Substitution test

Though X-bar clauses may seem arbitrary and unneeded, their existence can be confirmed by substitution. To the above sentence, "He studies linguistics at the university," someone could reply, "Oh, she does, too." The word "does," here, stands for the entire V-bar phrase, "studies linguistics at the university", thus implying the existence of this phrase as a complete unit of the whole sentence. In other words: if the V-bar phrase above were not defined as such, the sentence would have three separate phrases directly underneath S: the Noun phrase, the Verb phrase, and the Prepositional phrase. To substitute for two of them, together, as shown, implies that these two, together, make up one phrase within the sentence.

Reduction

In 1981, Tim Stowell tried to derive X-bar theory from more general principles in his MIT thesis Origins of phrase structure, a pathbreaking but ultimately unsuccessful effort according to Andras Kornai and Geoffrey Pullum³. In the mid 1990s, there were two major attempts to deduce versions of X-bar theory from independent principles. Richard Kayne's theory of Antisymmetry derived X-bar theory from the assumption that there was a tight relation between structure and linear order. Noam Chomsky's paper Bare Phrase Structure⁴ attempted to eliminate labelling (i.e. bar-levels) from syntax and deduce their effects from other principles of the grammar.

See also

• Antisymmetry
• Bare Phrase Structure
• Linguistics
• Minimalist syntax
• Phrase structure rules
• Syntax
• Sentence diagram

References

1. ^ Chomsky, Noam (1970). Remarks on nominalization. In: R. Jacobs and P. Rosenbaum (eds.) Reading in English Transformational Grammar, 184-221. Waltham: Ginn.
2. ^ Jackendoff, Ray (1977). X-bar-Syntax: A Study of Phrase Structure, Linguistic Inquiry Monograph 2. Cambridge, MA: MIT Press. 
3. ^ Kornai, Andras and Pullum, Geoffrey (1990) The X-bar theory of phrase structure. Language 66 24-50
4. ^ Chomsky, Noam (1994). "Bare Phrase Structure". MIT Occasional Papers in Linguistics (5). 

External links

• http://www.ucalgary.ca/~mcginnis/301/f04/Xbar.pdf
• http://www.ling.upenn.edu/~beatrice/syntax-textbook/index.html
• http://kornai.com/Papers/xbarver.pdf

Wikipedia content modification information:

• This page was last modified on 18 November 2008, at 11:24.

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