X-bar theory

The X-bar theory was first proposed by Noam Chomsky (1970). It postulates that all human languages share certain structural similarities, including the same underlying syntactic structure, which is known as the "X-bar".

X-bar

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

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

In the above:

• X is the head, the nucleus or the source of the whole syntactic structure, which is actually derived (or projected) out of it.
• 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 extranuclear, 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 UNLarium framework, there are eight different types of heads:

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

The heads define the nature of the phrase structures, thus:

• N projects a Noun Phrase (NP)
• V projects a Verbal Phrase (VP)
• J projects an Adjective Phrase (JP)
• A projects an Adverbial Phrase (AP)
• P projects a Prepositional Phrase (PP)
• D projects a Determiner Phrase (DP)
• I projects an Inflectional Phrase (IP)
• C projects a Complementizer Phrase (CP)

Noun Phrase	Verbal Phrase	Adjective Phrase	Adverbial Phrase	Prepositional Phrase	Determiner Phrase	Inflectional Phrase	Complementizer Phrase
NP / \ spec NB / \ NB adjunct / \ N comp | noun	VP / \ spec VB / \ VB adjunct / \ V comp | verb	JP / \ spec JB / \ JB adjunct / \ J comp | adjective	AP / \ spec AB / \ AB adjunct / \ A comp | adverb	PP / \ spec PB / \ PB adjunct / \ P comp | preposition	DP / \ spec DB / \ DB adjunct / \ D comp | determiner	IP / \ spec IB / \ IB adjunct / \ I comp | auxiliary verb	CP / \ spec CB / \ CB adjunct / \ C comp | conjunction

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), kind of (like), etc.
• frequency adverbs: always (go), never (go), usually (go), etc.
• negative adverbs: not (go)

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

etc.

Examples

NP	VP	JP	PP
NP / \ / \ / \ / NB / / \ spec head comp | | | D N PP | | | the construction of Babel	VP | VB / \ / \ / \ VB \ / \ \ head comp adjt | | | V N A | | | (he) bought something yesterday	JP / \ / \ / \ / JB / / \ spec head comp | | | A J PP | | | (I'm) really interested in Chemistry	PP | PB / \ head N | | without notice

Phrases

Specifiers, complements and adjuncts are themselves complex syntactic structures (XPs) 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	AP	I	VP	AP
CP	AP	C	IP	AP

Examples

construction	the construction	the fateful construction	the fateful construction of Babel
NP | NB | N | construction	NP / \ spec NB | | DP N | | DB construction | D | the	NP / \ spec NB | / \ DP adjt N | | | DB JP construction | | D JB | | the J | fateful	NP / \ spec NB | / \ DP adjt NB | | / \ DB JP N comp | | | | D JB construction PP | | | the J PB | / \ fateful P comp | | of NP | NB | N | Babel

Properties

Branching is binary

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

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 UNL^arium framework. They do not correspond to the current assumptions of the X-bar theory, and derive rather from extralinguistic issues (such as machine-tractability).

CP is the topmost level

In the UNL^arium framework, a clause is considered to be an instance of the complementizer phrase (CP), which is the syntactic topmost level.

    CP
   /  \
spec  CB
     /  \ 
    C    IP 
        /  \
     spec   IB
            / \ 
           I   VP

C (i.e., the head of CP) is to be left empty in case of independent clauses (i.e., not introduced by any conjunction).

CP / \ CB / \ IP | [Mary kissed Peter]

CP / \ CB / \ / \ / \ C IP | | (I know) that [Mary kissed Peter]

IP represents finite clauses

The Inflectional Phrase (IP) is always the complement of the Complementizer Phrase (CP) and represents finite clauses. In the UNL^arium approach, the head of IP is occupied only in case of compound tenses, i.e., by the auxiliary verb carrying information on tense, aspect and mood; in simple tenses, the head of the IP must be left empty.

    CP
   /  \
       CB
      /  \ 
          IP 
         /  \ 
             IB
            /  \ 
           I   VP
           |
       auxiliary

CP / \ CB / \ IP / \ IB / \ / \ / \ I VP | | [Mary kissed Peter]

CP / \ CB / \ IP / \ IB / \ / \ / \ I VP | | is [Mary kissing Peter]

CP / \ CB / \ IP / \ IB / \ / \ / \ I VP | | will [Mary kiss Peter]

CP / \ CB / \ IP / \ IB / \ / \ / \ I VP | | may [Mary kiss Peter]

etc.

Subjects

The subject of a clause is always represented at the position of spec of VP.

VP / \ Mary VB | [died]

VP / \ Mary VB | [kissed Peter]

VP / \ Mary VB | [gave a book to John]

VP / \ Mary VB | [is beautiful]

etc.

Objects, Adjuncts and Predicates

There is no structural difference between objects (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.

VB / \ buy NP | [a car]

VB / \ depend PP | [on me]

VB / \ go AP | [slowly]

VB / \ is JP | [beautiful]

etc.

Further reading

• Wikipedia