X-bar theory
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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 there are certain structural similarities among all phrasal categories of all languages, including one known as the "X-bar", which does not appear in traditional phrase structure rules.
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, or a P for preposition.
The term X-bar is derived from the notation representing this new structure. Certain structures are represented by 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.
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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 either order.
XP → (specifier), XTemplate:Unicode
XP XP
/ \ or / \
spec X' X' spec
2. One kind of X-bar consists of an X-bar and an adjunct, in either order.
(XTemplate:Unicode → XTemplate:Unicode, 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 either order.
XTemplate:Unicode → X, (complement...)
X' / \ X complement
(a head-first 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
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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
/ \
DetP N'
| |
Det' N
| |
Det cat
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the
The word the is a determiner (specifically an article), which is a type of specifier for nouns. Thus it is contained in the determiner phrase (DetP). The word cat is the noun which acts as the head of the noun phrase (NP).
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 and adjectives 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 different X-bar theory elements to different 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:
IP
/ \
NP I'
| | \
N I -----VP
| | |
He (present V'
tense) |
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-----------
/ \
V' PP
/ \ |
V NP P'
| | / \
studies N' P NP
| | / \
N at DetP N'
| | |
linguistics Det' N
| |
the university
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:
Missing image
Xbar_tree.png
X-Bar structure of a sentence
(This diagram uses the proper overbar notation.)
In this theory, the sentences is modeled as a verb phrase (VP). The noun phrase (NP) that is the subject of the sentence is located in 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 was 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.