X-bar theory

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The X-bar theory was first proposed by Noam Chomsky (1970)[1]. It postulates that all human languages share certain structural similarities, including the same underlying syntactic structure, which is known as the "X-bar". The Default Grammar is based in a modified version of the X-bar approach, as indicated below.

Contents

X-bar

The X-bar abstract configuration is depicted in the diagram below:

Xbar structure.png
  • X is the head, the nucleus or the source of the whole syntactic structure, which is actually derived (or projected) out of it. The letter X is used to signify an arbitrary lexical category (part of speech). When analyzing a specific utterance, specific categories are assigned. Thus, the X may become an N for noun, a V for verb, an J for adjective, a P for preposition, etc.
  • comp (i.e., complement) is an internal argument, i.e., a word, phrase or clause which is necessary to the head to complete its meaning (e.g., objects of transitive verbs)
  • adjt (i.e., adjunct) is a word, phrase or clause which modifies the head but which is not syntactically required by it (adjuncts are expected to be extra-nuclear, i.e., removing an adjunct would leave a grammatically well-formed sentence)
  • spec (i.e., specifier) is an external argument, i.e., a word, phrase or clause which qualifies (determines) the head
  • XB (X-bar) is the general name for any of the intermediate projections derived from X
  • XP (X-bar-bar, X-double-bar, X-phrase) is the maximal projection of X.


Constituents

The head, the complement, the specifier and the adjunct are said to be the constituents of the syntactic representation and define the four general universal syntactic roles.

Heads

In the X-bar diagram depicted above, the letter X is used to signify an arbitrary category. Thus, the X may become an N for noun, a V for verb, and so on. In the Default Grammar, there are eight different types of heads:

  • N = nouns and nominals: personal pronouns, demonstrative pronouns, nominalizations, etc
  • V = verbs
  • J = adjectives
  • A = adverbs[2]
  • P = prepositions
  • D = determiners: articles, demonstrative determiners, possessive determiners, quantifiers
  • I = auxiliary verbs
  • C = conjunctions

Specifiers

Specifiers are used to narrow the meaning intended by the head. They include:

  • articles: the (book), a (book), etc.
  • possessive determiners: my (book), your (book), etc.
  • demonstrative determiners: this (book), that (book), etc.
  • quantifiers: no (answer), every (hour), etc.
  • intensifiers (emphasizers, amplifiers, downtoners): very (expensive), quite (well), nearly (under), etc.

Complements

Complements are used to complete the meaning intended by the head. They may be:

  • direct objects: (do) something, (give) something
  • indirect objects: (laugh at) something, (give to) someone
  • complement of deverbals (i.e., nouns deriving from verbs): (construction of) the city, (arrival of) Peter
  • complement of adjectives: (loyal) to the queen, (interested) in Chemistry
  • complement of adverbs: (contrarily) to popular belief, (independently) from her
  • complement of prepositions: (under) the table, (after) today
  • complement of conjunctions: (and) Peter, (I don't know if) he'll come

Adjuncts

Adjuncts are used to modify the meaning intended by the head:

  • adjectives: beautiful (table)
  • adverbs: (speak) slowly
  • prepositional phrases: (table) of wood

Projections

In the X-bar theory, the heads (X) project two different types of structures:

  • XB (x-bar) is the intermediate projection, and is derived from the combination of the head or any of its intermediate projections with complements and adjuncts
  • XP (x-bar-bar, or x-phrase) is the maximal projection, and is derived from the combination of the topmost intermediate projection and the specifier

There can be as many intermediate projections as adjuncts and complements, but any head projects one single maximal projection, because it may have one single specifier.

The heads define the nature of the intermediate and maximal projections, thus:

  • A head N projects NB's and a Noun Phrase (NP)
  • A head V projects VB's and a Verbal Phrase (VP)
  • A head J projects JB's and an Adjective Phrase (JP)
  • A head A projects AB's and an Adverbial Phrase (AP)
  • A head P projects PB's and a Prepositional Phrase (PP)
  • A head D projects DB's and a Determiner Phrase (DP)
  • A head I projects IB's and an Inflectional Phrase (IP)
  • A head C projects CB's and Complementizer Phrase (CP)

Phrase structures

Specifiers, complements and adjuncts are themselves complex syntactic structures (i.e., maximal projections, or XP's) which are combined to form the sentence structure:

PHRASE STRUCTURE
XP
SPEC XB
XB ADJT
HEAD COMP
NP DP N PP JP,PP
VP NP V NP,PP PP,JP,AP
JP AP J PP AP,PP
AP AP A PP PP,AP
PP AP P NP,JP,VP AP
DP AP D AP
IP NP I VP AP
CP AP C IP AP

Examples

Tortoise.png
Thetortoise.png
Thebeautifultortoise.png
Thebeautifultortoisewontherace.png

Properties

Branching is binary

A key assumption of X-bar theory is that branching is always binary, if it occurs. This means that there should be as many XBs as complements and adjuncts.

  XP
  |
  XB
  |
 head
   XP
  /  \ 
spec XB
     |
    head
    XP
    |  
    XB
   /  \ 
head  comp 
    XP
    |  
    XB
   /  \ 
head  adjt 
   XP
  /  \  
spec  XB
     /  \ 
  head  adjt 
    XP
   / \
spec  XB
     / \
    XB  comp
   / \
  X   comp
  |
head
    XP
   / \
spec  XB
     / \
    XB adjt
   / \
  XB  comp
 / \
X   comp
|
head
etc.

Order is parametrized

The order of the constituents (specifiers, complements and adjuncts) is subject to language specific parametrization and may vary:

right complement left complement right adjunct left adjunct
    XP
    |  
    XB
   /  \ 
head  comp 
    XP
    |  
    XB
   /  \ 
comp  head 
    XP
    |  
    XB
   /  \ 
head  adjt 
    XP
    |  
    XB
   /  \ 
adjt head 
etc.

Conventions

The following conventions have been adopted in the Default Grammar. They do not correspond to the current assumptions of the X-bar theory, and derive rather from extralinguistic issues (such as machine-tractability).

Coordination

Coordination.png

In the original X-bar approach, branching is always binary. In the Default Grammar, this is also true, except for coordination, where branching is ternary. In any case, the coordinated constituents always project a structure of the same category (two coordinate NP's project a NP, two coordinated NB's project a NB, and so on). In case of a coordination of more than two constituents, the coordination must be represented in separate steps (i.e., branching cannot be greater than 3).

Surface and Deep Structures

Surfaceanddeepstructures.png

The Default Grammar differentiates between the surface syntactic structure and the deep syntactic structure. The former preserves the order of the words in the sentence; the latter preserves the dependency relations. The deep structure is converted into the surface structure, and vice-versa, through the movement of the constituents. This may entail different configurations for the same sentence, depending on the type of the representation.

Topmost levels

XP.png

The Default Grammar adopts the following general configuration, where CP is the topmost category, IP is the complement of CP, and VP is the complement of IP.

CP

CP.png

CP is the maximal projection of a conjunction. It is also used to represent topicalization (i.e., movement of a constituent out of its original position to the beginning of the sentence).
Differently from the current X-bar approach, the Default Grammar considers a clause to be an instance of CP (instead of DP). However, CP is represented only in two cases:

  • When there is a subordinating conjunction, which will be the head of CP; and
  • In the surface structure, when there is any topicalization. In this case, the topicalized constituent is represented at the position of adjunct of CP (even if the head of CP is empty).

CP is not represented when there is no subordinating conjunction or topicalized element.

IP

IP is the maximal projection of an auxiliary verb. Differently from the current X-bar approach, only auxiliary verbs may occupy the position of the head of IP. IP is represented in the following cases:

  • When the sentence contains an auxiliary verb, which will be the head of IP; and
  • When there is a CP and the sentence is finite (i.e., it is inflected in tense, aspect or mood).

IP is not represented when there is no CP nor auxiliary verb. The subject of a sentence is represented at the position of the spec of IP whenever the sentence contains an auxiliary verb. If this is not the case, and the subject is not topicalized, the subject is always represented at the position of the spec of VP.

VP

VP is the maximal projection of a main verb or a copula (but not of an auxiliary verb). It may contain one single specifier (the subject of the clause) and as many adjuncts and complements as necessary. The position of the spec of VP is occupied only if there is no auxiliary verb (in this case the subject is represented as the spec of IP) or when the subject is not topicalized (in this case it is represented as the adjunct of CP). There is no structural difference between complements (either direct or indirect) or adjuncts. They are always represented as branches of the intermediate projection. Predicates are represented as complements of copula (linking) verbs.

NP

NP is the maximal projection of a noun. It may contain one single specifier (DP), and as many adjuncts and complements as necessary.

JP

JP is the maximal projection of an adjective. It may contain one single specifier (AP), and as many adjuncts and complements as necessary.

AP

AP is the maximal projection of an adverb. It may contain one single specifier (other AP), and as many adjuncts and complements as necessary.

PP

PP is the maximal projection of a preposition. It may contain one single specifier (AP), and as many adjuncts and complements as necessary.

DP

DP is the maximal projection of a determiner. It may contain one single specifier (other DP) and adjuncts. A DP may not contain complements.

Further reading


Notes

  1. Chomsky, Noam (1970). Remarks on nominalization. In: R. Jacobs and P. Rosenbaum (eds.) Reading in English Transformational Grammar, 184-221. Waltham: Ginn.
  2. In the X-bar theory, differently from the UNLarium approach, adverbs are subsumed by prepositions and are not considered to be an independent lexical category.
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