What are the principles of X-bar theory?
What are the principles of X-bar theory? What is its justification in syntactic theory?
Noam Chomsky has been a pivotal twentieth- and twenty-first-century theorist in the field of linguistics, proffering several hypotheses and philosophies to shape current trends in research, including generative grammar and the eponymous Chomsky hierarchy. His ideas have also been significant in the areas of government and binding theory, transformational grammar and context-free grammar. It perhaps comes as no surprise, then, that Chomsky was also the initial proposer of X-bar theory, a focal point of linguistic theory, even though much of the further development was undertaken by Ray Jackendoff.
This essay will explore the principles and importance of X-bar theory, and how it fits overall into other elements of syntactic theory. As a revolutionary way of describing syntax, X-bar theory builds on basic phrase structure theories that have come before it and changed how linguists view syntactic models today. I will therefore describe some basic phrase structure rules before tracing their evolution through to X-bar theory, before culminating in an explanation of X-bar theory's overall significance and its general position in syntactic studies.
Phrase structures are used in linguistics to act as an almost mathematical formula that has the power to summarise phrase structures in any language, and their syntactic constituents, succinctly and precisely. The constituent parts comprise phrasal categories and lexical categories, the former consisting of noun phrases, verb phrases and prepositional phrases, and the latter consisting of noun, verb, adjective, and adverb, among others. Phrase structure rules, when written 'conventionally', consist of an equalised structure whereby the elements on one side of the arrow are equivalent to the elements on the other side, such as:
S -> NP VP
NP -> Det N1
N1 -> (AP) N1 (PP)1
The elements on the right-hand side, as is perhaps self-evident, are sub-categories of the elements depicted on the left-hand side of the equation. The major flaw of basic phrase structure rules, however, is perhaps that grammatically sound constructions are possible when using phrase structure trees without the constructions needing to make semantic sense, as shown by Chomsky's famous example of a nonsensical sentence construction2:
As well as the above demonstration that phrase structure rules can be somewhat redundant if they can represent phrases that make little or no semantic sense, it would also be time-consuming if each language's phrase structure had to be referred to by way of an individual phrase structure tree, even if they possessed shared syntactic qualities. X-bar theory therefore exhibits its relevance here, as it allows the qualities of languages' syntax to be exhibited via similar tree structures, while at the same time allowing languages that share syntactic features to also share a tree and thus be represented concurrently or simultaneously.
Where NP, VP, N, V etc would be used in normal phrase structure trees, X-bar theory uses an X(P) in this place to signify an arbitrary lexical category. XP, or X-phrase, when written, is equal to X'', or X-bar-bar/X-double-bar, as represented by the double overbar. Elements such as N, V, A and P are represented as X', or X-bar, with just one overbar, overbars generally indicating how embedded the element is within the given phrase. The use of X means that our grammar need not contain four schemata when only one schema is required.3 The general format for ...
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Where NP, VP, N, V etc would be used in normal phrase structure trees, X-bar theory uses an X(P) in this place to signify an arbitrary lexical category. XP, or X-phrase, when written, is equal to X'', or X-bar-bar/X-double-bar, as represented by the double overbar. Elements such as N, V, A and P are represented as X', or X-bar, with just one overbar, overbars generally indicating how embedded the element is within the given phrase. The use of X means that our grammar need not contain four schemata when only one schema is required.3 The general format for phrase structure is summarised in the following phrase-structure rules:
. An X-phrase consists of an optional specifier and an X-bar, in any order, e.g.:
XP XP
/ \ or / \
spec X' X' spec
2. One kind of X-bar contains an X-bar and an adjunct, again in any order:
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 (this number can be zero).
X' X'
/ \ or / \
X complement complement X
X-bar theory therefore brings out what is common in the structure of phrases. All phrases are controlled by a head, the head being a terminal node that dominates the words in the phrase. Complements and adjuncts combine with X' to form X' projections. It is worth mentioning, however, a crucial difference between adjuncts and complements when they are used in X-bar theory: adjuncts are iterative (that is, they show the property of recursivity) and can thus generate infinite strings of nodes, whereas complements do not possess this quality (as on the left side of the rule there in an X', but on the right-hand side there is only an X).4 Specifiers combine with the topmost X' to form the maximal projection XP. As Jackendoff points out, without X-bar theory there is no way to capture cross-category generalisation directly.5
It is worth mentioning that different grammars express X-bar theory differently, whereby X-bar theory elements are assigned to phrase types in different ways. A transformational grammar may parse a phrase like this:
In this example, the IP is the inflectional phrase, and its specifier is the noun phrase (NP), while the complement is the verb phrase (VP). However, a head-driven phrase structure grammar might model the sentence as a verb phrase, with a noun phrase still acting as a specifier.
X-bar theory, unlike regular phrase structure rules, has the expression of general principles, rather than irregularities, at its heart. It captures the properties of all phrases and provides an endocentric model for generative grammar,6 and as well as following on quite conveniently from general phrase-structure theories, it also gave rise to Richard Kayne's theory of antisymmetry and Chomsky's "revision" of X-bar theory, bare phrase structure. The latter attempted to eliminate labelling (that is, bar levels) from syntax and deduce their effects from other principles of the grammar.
Kayne created antisymmetry as a derivation of X-bar theory. As with Chomsky's original X-bar theory, the underlying principle of antisymmetry is the notion of a universal hierarchical structure, using a branching order of specifier-head-complement. He goes on to hypothesise that any variation from this structure indicates that movement has gone on to disrupt the underlying order.7 It is primarily based on the idea of c-command (a relationship between nodes in a phrase structure tree), although antisymmetry differs from X-bar theory by diverging somewhat from simplicity. Kayne makes a distinction between segments and categories, whereby a category is an extended node and if two connected nodes in a tree have the same label, they are two segments of the same category. C-command is defined in terms of categories using the notion of exclusion: if a category is not dominated by both of its segments, then it is excluded. If every category that dominates A also dominates B, then A c-commands B and so cancels B out, as shown by the following diagram:
If A c-commands B but the reverse is untrue, then the asymmetric c-command comes into play (whereby the asymmetric c-command is what holds A and B together in such a scenario). The tree example given above shows how antisymmetry 'agrees' with X-bar theory (with the exception that [Spec, CP] is treated as an adjunct). Removing any of the structure in the tree destroys the c-command that Kayne deems necessary for linearisation. Antisymmetry therefore clearly assumes from X-bar theory that there is a close relationship between structure and linear order.
We have seen how X-bar theory fits in syntactic theory between basic phrase structure rules and Kayne's theory of antisymmetry. The three theories could be viewed as something of a hierarchical structure in themselves: X-bar theory attempts to resolve some of the inadequacies and flaws associated with basic phrase structure rules, and antisymmetry attempts to further distil the principles of X-bar theory. However, I do not believe that antisymmetry fully achieves its aims, owing to the further 'complication' of the introduction of the c-command.
However, the apparent simplicity and advantages of X-bar theory have by no means rendered it immune to criticism. Andras Kornai and Geoffrey Pullum, in their 1990 paper entitled "The X-Bar Theory Of Phrase Structure", openly declare that they find X-bar theory to be insubstantial in its claims, and that X-bar theory violates various principles such as lexicality and succession, though it appears that part of the problem lies in the sheer number of explanations and expansions of X-bar theory. The authors also point out that relatively minor changes in X-bar theory can have catastrophic linguistic effects, as the X-bar grammar languages are not closed under finite state transductions (an example they give is the morphological substitution of de le for du in French).8 Nevertheless, the authors do agree that the key concept of X-bar theory, headedness, is relatively stable, and they concede at the end of the paper that they do still see a future for X-bar theory, albeit with some major revisions made to it.
Despite these various objections, though, X-bar theory still holds a prominent position in the field of generative grammar, and will assumedly serve as a talking point among grammarians for the foreseeable future. As previously stated, some grammarians consider X-bar theory to be outmoded and perhaps even erroneous in places, but just as many disagree. X-bar theory clarifies and simplifies core principles of phrase structure grammars, allowing generalisations to be made across various languages by using a traditional mathematical rule: the use of X to represent any given element so that phrase structures (or different methods of 'coding' language) across languages can be expressed succinctly. This also allows formulae to be created to express linguistic hypothesis, whereas basic phrase structure rules may require more specific knowledge before hypotheses can be advanced for consideration.
X-bar theory also sits neatly in the timeline of linguistic theory between the formation of basic phrase structure rules and the creation of Kayne's theory of antisymmetry. This close association with other theories allows it to be considered in relation to them on the basis of its merits and failings and used accordingly in phrase structure research according to those merits and failings. The fact that X-bar theory allows generalisation across languages where basic phrase structure rules perhaps do not is not to be underestimated; equally, the added dimension of the c-command in the theory of antisymmetry permits a greater range of linguistic perspective. Perhaps more significantly, the linear approach of X-bar theory presents a deviation from what could perhaps be called the variationist perspective of basic phrase structure rules: X-bar theory allows linguists to operate on a basis of generalisations, or rules plus exceptions, to a far greater extent than is usual in the two theories immediately related to it, where different syntactic methods of producing the same phrase are often studied.
--Works cited-
Carnie, A., Syntax: A Generative Introduction, Blackwell Publishing, 2002
Chomsky, N., Syntactic Structures, The Hague/Paris: Mouton, 1957
Cook, V. J., Chomsky's Universal Grammar: An Introduction, Oxford: Blackwell Publishing, 1988
Haegeman, Liliane, Introduction to Government and Binding Theory, Oxford: Blackwell Publishing, 1994
Jackendoff, R., X' Syntax: A Study of Phrase Structure, MIT Press: Cambridge, MA, 1977
Kayne, Richard S., The Antisymmetry of Syntax. Linguistic Inquiry Monograph Twenty-Five, MIT Press, 1994
Kornai, A., and Pullum, G. K., "The X-Bar Theory of Phrase Structure", Language, 1990, pp. 24-50
--Works consulted--
Chomsky, N., Some Concepts and Consequences of the Theory of Government and Binding, Cambridge, Mass; London: MIT Press, 1982
Chomsky, N., "Remarks on nominalization", Readings In English Transformational Grammar, Jacobs, R., and Rosenbaum, P. (eds), Ginn, 1970
Chung, S., "Current Generative Approaches to Syntax", Annual Review of Anthropology, 1987, pp. 179-96
Gazdar, G., "On Syntactic Categories", Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 1981, pp. 267-83
Schwabe, K., and Winkler, S., The Interfaces: Deriving and Interpreting Omitted Structures, Amsterdam: J Benjamin's Publishing Company, 2003
Smith, N., Chomsky: Ideas and Ideals, Cambridge University Press, 1999
Whereby S represents a grammatical sentence, NP represents a noun phrase, VP represents a verb phrase etc.
2 Chomsky, Noam, Syntactic Structures, The Hague/Paris: Mouton, 1957, p. 15
3 Haegeman, Liliane, Introduction to Government and Binding Theory, Oxford: Blackwell Publishing, 1994, pp. 103-5
4 Carnie, A., Syntax: A Generative Introduction, Blackwell Publishing, 2002, p. 120
5 Jackendoff, R., X' Syntax: A Study of Phrase Structure, MIT Press: Cambridge, MA, 1977, p. 18
6 Cook, V. J., Chomsky's Universal Grammar: An Introduction, Oxford: Blackwell Publishing, 1988
7 Kayne, Richard S., The Antisymmetry of Syntax. Linguistic Inquiry Monograph Twenty-Five, MIT Press, 1994
8 Kornai, A., and Pullum, G. K., "The X-Bar Theory of Phrase Structure", Language, 1990, pp. 24-50, p. 47
