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X-bar theory

SYNTAX: theory about the internal structure of syntactic constituents which was originally intended to place constraints on the power of phrase structure rules. X-bar theory captures the insight that all phrases share some essential structural properties. EXAMPLE: the structures in (i) have in common that the head (noun, verb, adjective, or preposition) has an element to its right, which can be construed as its complement.

(i)  a lookV [for you]
     b the searchN [for you]
     c angryA [with you]
     d onP [with you]
These structural properties are conventionally represented as in (ii).
(ii)            X''
	       / \
              /   \
	specifier  X'
                  / \
                 /   \ 
		X   complement
In (ii), X is called the head of the phrase. X' and X'' are called projections of X. Typographically, these projections are marked by one or more primes (X' and X''), called bars. Thus, X' is pronounced X-bar; X'', X-double-bar etc.. The head is called the zero projection (also written as X0). The topnode X'' (or XP) is called the maximal projection of X. All other projections between the head and the maximal projection are called intermediate projections. The sister(s) of X are called the complements of the head), and the sister(s) of X' is/are the specifier(s) (of the phrase). The structure in (ii) embodies the following assumptions about the internal structure of constituents:
- Each phrase XP has one unique, structurally obligatory
element: the head X0 of the phrase. 
- There is one maximal projection, or X-max, corresponding 
to the phrasal node (XP). There is no strong consensus about 
whether the X-max node always has two bars.
- All other elements within the XP besides the head are 
structurally optional, their presence being determined by 
principles of  licensing (  theta-theory,  case theory). 
They can appear both on the left and on the right hand side 
of the head.
- The intermediate projection X' between the head and the 
maximal projection can be iterated.
- In case of  adjunction, the maximal projection XP can 
also be iterated.
- Except for the possibilities of iteration, projections 
must obey condition (iii):
(iii)	Xn  ->  Xn-1
This condition excludes the possibility of endless projection loops, i.e. phrases like the one in (iv):
(iv)		X''
               / \
The value of X ranges over at least the categories N, A, V, P, the so-called lexical categories (see lexical category). These categories can be characterized as follows in terms of the features [±N] (substantive) and [±V] (predicative):
(v) a	[+N,+V]	=	A
    b	[+N,-V]	=	N
    c	[-N,+V]	=	V
    d	[-N,-V]	=	P
This notation allows crosscategorical reference to sets of categories. EXAMPLE: the two categories that can appear with an NP complement in English, Verbs and Prepositions, can now be referred to with the single feature [-N]. Thus, it becomes possible to characterize natural classes of syntactic categories. The X-bar system can be extended to clausal structures. Functional categories like I ( INFL) and C ( COMP) are also considered to be values of X. In this way, S is reinterpreted as a projection of I(NFL); S' is reinterpreted as a projection of C(OMP). Analogously, the functional category Determiner (D) can be incorporated into the X-bar system. In this way, NP is reinterpreted as being part of a DP.
LIT. Abney, S. (1987)
Chomsky, N. (1994)
Chomsky, N. (1993)
Chomsky, N. (1986b)
Chomsky, N. (1986a)
Chomsky, N. (1970)
Jackendoff, R. (1977)