20 X-Bar Theory
Mr. Abhijit Debnath
Introduction:
Among the several modules of Government and Binding theory, X–Bar theory is a comprehensive module providing the description for a structural representation of the complex relationships between the various grammatical categories in a language that combine in a sequence to form a phrase, clause or a sentence. In short the proper sequence of words, that human grammar restricts all sentences to, need to follow a grammaticality constraint. But these kinds of constraints can be better studied and formulated only if a proper and precise relational representation is prepared to highlight and clarify at some symbolic representation, the complex links between each word of a sentence, existing directly or indirectly. X–Bar theory represented by X–Bar Schema does exactly that. It allows formulation of several levels of complex relationships between any category and the lexical item contained in it by avoiding redundancy, improving representational adequacy for all constituents and allowing for the appropriate accommodation of all kinds of complex relationships between each constituent of a phrase, clause or a sentence. In addition, the implementation of X-Bar theory also allows proper solutions for a lot of computational needs that demands the representation of the structural complexity of human language for the purpose of dealing with Natural Language Processing and automation of sentence generation, parsing and several significant breakthroughs.
Advent of the theory:
X- Bar theory came to be formulated as a better implementation of the relationships between the nodes of a syntactic structure. Previous to it, Phrase Structure was developed to represent noun phrases, verb phrases, adjective phrases and so on. But the relationship between a word of that phrase and another phrase could not be represented by any means. e.g., there was nothing to represent the property of a noun phrase representing whether the noun or the determiner or even an adjective modifying it should be somehow related to the verb in a verb phrase where the relation of semantic roles as well as case has to be built between the verb and one of the categories in the noun phrase. The amount of branching was also not controlled as addition of adverbs, multiple genitive constructions etc., would give rise to several amounts of branching from the top most phrase at the Sentence level representation. Phrase structure rules also implied that one needs to create different rules each for each type of phrase such as NP, VP, PP etc. as the representation did not distinguish between an integral category in the phrase and the other obligatory or optional categories inside it. Such redundancies have been adequately dealt with in the X-Bar approach to structural representation of the intricate sequential relationship between words of a language that shape a grammatical as well as meaningful sentence. The following discussion will clarify the preciseness of the approach in detail.
The part of the grammar regulating the structure of phrases has come to be known as X’ theory. Here the representation of lexical properties of a word at a phrasal level is endocentric in nature and those properties are very strategically represented at a compositional level higher than the level of its entry in the structure at the phrasal level.
The following is the basic X-Bar Schema according to the proposals of Chomsky (1970)
Therefore, all phrases are conceptually headed by one head, X. The head of the projection is a zero projection (X0). Heads are terminal nodes: they dominate words. X’ theory distinguishes two further levels of projection. Complements combine with X to form X’-projections (3), adjuncts combine with X’ to form X’ projections (2). The specifier combines with the topmost X’ to form the maximal projection XP (1).
There are however limitations to what can be taken as a complement or a specifier by the head depending on whether it is a verb, a noun, a preposition or an adjective and so on. These restrictions are actually explained by other modules of Government and Binding such as Case theory, Theta Criteria and so on besides several choice based restrictions upon the head such as L-Selection, S-Selection and C-Selection and are however not an integral part of the X-Bar module itself.
As evident from Figure 1, the Specifier (Spec) is the sister node to X’ and together form XP. The complement YP is also a sister to the head X. The Adjunct ZP can combine with only an X’ to form X’ again.
The bars can also be understood as a projection of the properties of X. Therefore X’s are all intermediate projections of X and X” or XP is the maximal projection of X. There can be several intermediate projections of X in the form of X’. This schema helps accommodate several complements of X such as double object constructions for give which must take two objects, the direct object the book and the recipient him for the book in John gave him the book. This can be represented as follows.
This schema also helps accommodate several additions of adjuncts or extra information to the verb which may not be integral to or necessary for the comprehension of the given expression. Adjuncts such as yesterday, in the library and so on can be added to the sentence in Figure 2 in the following manner.
Salient Features:
The X’-format allows us to bring out the commonality between the different types of phrases. The traditional phrase structure rules for specific phrases, eg VP etc., are reduced to more elementary notions. The hierarchical organization of the phrase is captured by X’ -theory. The X’-schema can be extended to the clausal constituents: S is reinterpreted as a projection of INFL, with the subject NP in the specifier position. S’ is reinterpreted as a projection of C.
Head
The Head of a phrase X0, is the category with which the other lexical categories in the phrase are related in such a way that the head bears the essence of the meaning of that phrase. It is the word that determines the syntactic type of a phrase. Heads are the always terminal nodes in a phrase below which the lexical insertion takes place. The head of hot tea is tea where tea is a noun and the phrase is considered as a noun phrase. hot however is an adjective and does not get to categorize the phrase as an adjective phrase. If the lexical head is a verb, it can assign certain roles like theta roles, case etc. to its NP arguments and so on. Nouns, verbs, prepositions, Adjectives as well as Infl and so on can be heads.
Complement
The complement rule X’ → X0; YP says that a complement phrase YP can be the sister of the terminal head node X0 to constitute an intermediate projection X’. Complements are those words which the head can get to choose according to its lexical selection, constituent selection and semantic selection requirements. These are the arguments which become the environment in which the head can occur as a chunk. Complements are also arguments of the head and necessary to complete the meaning of a given
expression and help complete the meaning of a phrase. E.g., The object Sam of a verb hit is considered its complement in John hit Sam. The noun phrase the table inside a preposition phrase on the table is a complement of the preposition on. Complements are sister node to X0 under which the actual lexical insertion of a word takes places.
Specifier
The specifier rule XP → Spec; X’ says that a specifier Spec can be the sister to the intermediate projection X’ to combine into a constituent of the maximal projection XP. Therefore specifier is a word added at the XP or X” level as its sister node. Mostly the determiners such as the have been considered as Specifier of a phrase. Therefore modifiers of NP have been given the position of a specifier. The subject of a sentence is always given the position of a specifier (Spec of IP).
Adjunct
The adjunct rule X’*→ X’; YP states that an adjunct YP can be sister to only an intermediate node X’ and never with any other node, to form an intermediate node X’. Unlike other nodes, adjunct is an extra information and is not necessarily integral to the basis comprehension of the given expression. The addition of words like the adverb yesterday and the prepositional phrase in the library etc can be done away with and still the basic meaning of give can be completely represented solely with the objects beside it.
The structural relations c-command and m-command are some of the crucial concepts that help define the connections between the relevant types of nodes.
C-command
A c-commands B if and only if A does not dominate B and every X that dominates A also dominates B.
When X is equated with the first branching node, A c-commands B. When X is interpreted as a maximal projection, A m-commands B.
Government
A governs B if and only if
-
A is a governor;
- A m-commands B; and
- no barrier intervenes between A and B.
Maximal projections are barriers to government. Governors are heads.
Categorical Representation with X-Bar:
Structuring the phrasal categories according to X’-schema provided certain changes to the existing phrase structure rules in the Transformational Generative Grammar. It not only economized the representational adequacy but also brought about certain new categorical representations in use.
Earlier S was the head of the whole sentence but now Infl (inflection) became the head of the whole sentence. S’, the subordinate clause beginning with that was converted to CP or complement phrase with a C (complement) head. This would have an Infl Phrase or IP as a complement and the sentence representation would follow. The following is an appropriate example to highlight the difference.
As the comparison in Figure 4 suggests, the tree on the left depicts the whole sentence having its representation by an arbitrary symbolic representation of S and the embedded clause Mary loves icecream has S’ as the head, where as the tree on the right uses X’-Schema to represent the Infl as the essence of the matrix clause and represents it in an endo-centric approach at the maximal level of the sentence. The embedded clause is represented as a complement phrase with the comp that as the head. Further within the complement phrase is the IP representing a full sentence again with inflection as the essence of representation at the maximal level. The spec position of CP is however left unoccupied and provides a site for transformational requirements such as Wh-movement in case the object of love is what where the it moves out of the NP position leaving a trace in Figure 5 as follows. The subscript “i” indicates the identity of the trace and the Spec what as being the same.
The justification of X-bar theory also requires as an integral Projection Principle which states that every lexical property must be projected till the maximal level of representation. Therefore X0 node with the lexical entry of any noun beneath it must keep projecting the property of the noun through all intermediate X’ nodes till it reaches the maximal level X” or XP. Therefore the properties of the noun ice-cream must be projected through the head X0 level rendering it a N0, through any number of intermediate representations of X’ rendering them as N’s till the maximal X” or XP level, rendering it as NP. Same will be the case with verbs, prepositions, adjectives and so on. The complement of the verb.
Significance and later developments:
X-bar theory was first proposed by Noam Chomsky (1970), building on the approach to categories Zellig Harris (1951). These are important developments in the area of binary branching and which captures the composition of all categories in a very efficient manner, attempting to also represent the psychological level of representation of the awareness of linguistic units in our mind to a great extent. X-bar theory was incorporated into both transformational and non-transformational theories of syntax, including GB, GPSG, LFG, and HPSG. The development of Minimalist Program has largely abandoned X-bar theory and adopted the approaches of Bare Phrase Structure which operates with less amount of structure and has changed the initial approach of Chomsky towards the tradition of Dependency Grammar.
| you can view video on X-Bar Theory | |
References:
- Black, C. (1999). A step-by-step introduction to the Government and Binding theory of syntax (1st ed.). Summer Institute of Linguistics. Retrieved from http://www.sil.org /americas/ mexico/ling/ E002- IntroGB.pdf
- Brown, E. (2006). Encyclopedia of language and linguistics. Amsterdam: Elsevier.
- Chomsky, N. (1957). Syntactic structures. The Hague: Mouton.
- Chomsky, N. (1965). Aspects of the theory of syntax. Cambridge: M.I.T. Press.
- Chomsky, N. (1993). Lectures on government and binding. Berlin: Mouton de Gruyter.
- Chomsky, N. (1995). Language and Nature. Mind, 104(413), new series, 1-61. Retrieved from http://www.jstor.org/stable/2254605
- Culicover, P. (1997). Principles and parameters. Oxford: Oxford University Press.
- Haegeman, L. (1994). Introduction to government and binding theory. Oxford, UK: B. Blackwell.
- Lasnik, H. & Uriagereka, J. (1988). A course in GB syntax. Cambridge, Mass. [u.a.]: MIT Pr.