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| '''Phrase structure rules''' are a way to describe a given language's [[syntax]] and are closely associated with the early stages of [[Transformational Grammar|transformational grammar]].<ref>Phrase structure rules were proposed first by Chomsky (1957).</ref><ref>For general discussions of phrase structure rules, see for instance Borsley (1991:34ff.), Brinton (2000:165), Falk (2001:46ff.).</ref> They are used to break down a natural [[language]] sentence into its ''constituent parts'' (also known as [[syntactic categories]]) namely [[syntactic category|phrasal categories]] and [[syntactic category|lexical categories]] (aka [[part of speech|parts of speech]]). A grammar that uses phrase structure rules is a type of [[phrase structure grammar]] - except in [[computer science]], where it is known as just a [[Formal grammar|grammar]], usually [[context-free grammar|context-free]]. Phrase structure rules as they are commonly employed operate according to the constituency relation and a grammar that employs phrase structures rules is therefore a ''constituency grammar'' and as such, it stands in contrast to [[dependency grammar]]s, which are based on the dependency relation.<ref>Dependency grammars are associated above all with the work of Lucien Tesnière (1959).</ref>
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| ==Definition==
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| Phrase structure rules are usually of the following form:
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| :<math>A \to B \quad C</math>
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| meaning that the [[constituent (linguistics)|constituent]] <math>A</math> is separated into the two subconstituents <math>B</math> and <math>C</math>. Some further examples for English are as follows:
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| :<math>S \to NP \quad VP</math> | |
| :<math>NP \to Det \quad N1</math>
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| :<math>N1 \to (AP) \quad N1 \quad (PP)</math>
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| The first rule reads: An S ([[Sentence (linguistics)|sentence]]) consists of an NP ([[noun phrase]]) followed by a VP ([[verb phrase]]). The second rule reads: A noun phrase consists of a Det (determiner) followed by an N (noun). Some further categories are listed here: AP ([[adjective phrase]]), AdvP ([[adverb phrase]]), PP ([[prepositional phrase]]), etc. Specific notations for writing phrase structure rules can be identified in the third rule. The round brackets around AP and PP indicate that these constituents are optional. A single noun can still stand as a noun phrase.
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| Applying the Phrase Structure Rules in a neutral manner, it is possible to generate many proper sentences of English. But it is also quite possible that the rules generate syntactically correct but semantically nonsensical sentences. The following example sentence is notorious in this regard, since it is complete nonsense, even though it is syntactically correct:
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| ::[[Colorless green ideas sleep furiously]]
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| This sentence was constructed by [[Noam Chomsky]] as an illustration that Phrase Structure Rules are capable of generating syntactically correct but semantically incorrect sentences. Phrase structure rules break sentences down into their constituent parts. These constituents are often represented as tree structures. The tree for Chomsky's famous sentence can be rendered as follows:
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| :[[image:cgisf-tgg.svg|300px|Colorless green ideas sleep furiously.]]
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| A constituent is any word or combination of words that is dominated by a single node. Thus each individual word is a constituent. Further, the subject NP ''Colorless green ideas'', the minor NP ''green ideas'', and the VP ''sleep furiously'' are constituents. Phrase structure rules and the tree structures that are associated with them are a form of [[immediate constituent analysis]].
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| ==Top down== | |
| An important aspect of phrase structure rules is that they view sentence structure from the top down. The category on the left of the arrow is a greater constituent and the immediate constituents to the right of the arrow are lesser constituents. Constituents are successively broken down into their parts as one moves down a list of phrase structure rules for a given sentence. This top-down view of sentence structure stands in contrast to much work done in modern theoretical syntax. In [[Minimalist program|Minimalism]]<ref>See for instance Chomsky (1995).</ref> for instance, sentence structure is generated from the bottom up. The operation [[Merge (linguistics)|Merge]] merges smaller constituents to create greater constituents until the greatest constituent (i.e. the sentence) is reached. In this regard, theoretical syntax abandoned phrase structure rules long ago, although their importance for [[computational linguistics]] seems to remain intact.
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| ==Alternative approaches==
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| ===Constituency vs. dependency===
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| Phrase structure rules as they are commonly employed result in a view of sentence structure that is ''constituency-based''. Thus grammars that employ phrase structure rules are ''constituency grammars'' (=[[phrase structure grammar]]s), as opposed to ''[[dependency grammar]]s'',<ref>The most comprehensive source on dependency grammar is Ágel et al. (2003/6).</ref> which view sentence structure as ''dependency-based''. What this means is that for phrase structure rules to be applicable at all, one has to pursue a constituency-based understanding of sentence structure. The constituency relation is a one-to-one-or-more correspondence. For every word in a sentence, there is at least one node in the syntactic structure that corresponds to that word. The dependency relation, in contrast, is a one-to-one relation; for every word in the sentence, there is exactly one node in the syntactic structure that corresponds to that word. The distinction is illustrated with the following trees:
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| :[[File:Phrase structure rules.jpg|Phrase structure rules: Constituency vs. dependency]]
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| The constituency tree on the left could be generated by phrase structure rules. The sentence S is broken down into smaller and smaller constituent parts. The dependency tree on the right could not, in contrast, be generated by phrase structure rules (at least not as they are commonly interpreted).
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| ===Representational grammars===
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| A number of representational phrase structure theories of grammar never acknowledged phrase structure rules, but have pursued instead an understanding of sentence structure in terms the notion of [[Schema (psychology)|schema]]. Here phrase structures are not derived from rules that combine words, but from the specification or instantiation of syntactic schemata or configurations, often expressing some kind of semantic content independently of the specific words that appear in them. This approach is essentially equivalent to a system of phrase structure rules combined with a non[[compositionality|compositional]] [[semantics|semantic]] theory, since grammatical formalisms based on rewriting rules are generally equivalent in power to those based on substitution into schemata.
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| So in this type of approach, instead of being derived from the application of a number of phrase structure rules, the sentence ''Colorless green ideas sleep furiously'' would be generated by filling the words into the slots of a schema having the following structure:
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| ::[NP[ADJ N] VP[V] AP<nowiki>[[ADV]]</nowiki> | |
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| And which would express the following conceptual content:
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| ::X DOES Y IN THE MANNER OF Z
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| Though they are noncompositional, such models are monotonic. This approach is highly developed within [[Construction grammar]]<ref>Concerning Construction Grammar, see Goldberg (2006).</ref> and has had some influence in [[Head-Driven Phrase Structure Grammar]]<ref>Concerning Head-Driven Phrase Structure Grammar, see Pollard and Sag (1994).</ref> and [[Lexical Functional Grammar]],<ref>Concerning Lexical Functional Grammar, see Bresnan (2001).</ref> the latter two clearly qualifying as phrase structure grammars.
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| ==See also==
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| {{div col|cols=3}}
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| *[[Constituent (linguistics)|Constituent]]
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| *[[Dependency grammar]]
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| *[[Immediate constituent analysis]]
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| *[[Phrase]]
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| *[[Phrase structure grammar]]
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| *[[Syntactic category]]
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| {{div col end}}
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| ==Notes==
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| {{Reflist|2}}
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| ==References==
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| {{refbegin|2}}
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| *Ágel, V., Ludwig Eichinger, Hans-Werner Eroms, Peter Hellwig, Hans Heringer, and Hennig Lobin (eds.) 2003/6. Dependency and Valency: An International Handbook of Contemporary Research. Berlin: Walter de Gruyter.
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| *Borsley, R. 1991. Syntactic theory: A unified approach. London: Edward Arnold.
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| *Bresnan, Joan 2001. Lexical Functional Syntax.
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| *Brinton, L. 2000. The structure of modern English. Amsterdam: John Benjamins Publishing Company.
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| *Carnie, A. 2013. Syntax: A Generative Introduction, 3rd edition. Oxford: Blackwell Publishing.
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| *Chomsky, N. 1957. Syntactic Structures. The Hague/Paris: Mouton.
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| *Chomsky, N. 1995. The Minimalist Program. Cambridge, Mass.: The MIT Press.
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| *Falk, Y. 2001. Lexical-Functional Grammar: An introduction to parallel constraint-based syntax. Stanford, CA: CSLI Publications.
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| *Goldberg, A. 2006. Constructions at Work: The Nature of Generalization in Language. Oxford University Press.
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| *Pollard, C. and I. Sag 1994. Head-driven phrase structure grammar. Chicago: University of Chicago Press.
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| *Tesnière, L. 1959. Éleménts de syntaxe structurale. Paris: Klincksieck.
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| [[Category:Grammar frameworks]]
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