Phrase structure rules

Phrase-structure rules are used in transformational-generative grammar (TGG) to describe a given language's syntax. This is accomplished by attempting to break language down into its constituent parts (also known as syntactic categories) namely phrasal categories and lexical categories (aka parts of speech). Phrasal categories include the noun phrase, verb phrase, and prepositional phrase; lexical categories include noun, verb, adjective, adverb, and many others.

There are many kinds of phrase-structure rules (PSRs), which themselves can be combined to generate additional PSRs. In particlar, TGG and PSRs must account for the following characteristics:

1. All languages combine nouns (N) and verbs (V) to express ideas about the universe. However, some languages do not distinguish nouns and verbs as separate classes.
2. All languages have rules determining how these are combined into meaningful units.
3. All languages have recursion, i.e. at least one rule that can be repeated ad infinitum:
   1. An example of this is the English use of "and", which can link any series of two or more nouns or two or more verbs:
      1. "His and hers and theirs and Mary's and John's... etc. "
      2. "He ran and jumped and played and skipped and danced and ... etc. "
   2. This would be described in TGG as:
      1. A noun phrase (NP) consists of a N or NP, the word and, and another N or NP.
      2. A verb phrase (VP) consists of a V or VP, the word and, and another V or VP.

TGG, essentially, attempts to set down or categorize all the rules of a language that lead to grammatical utterances, regardless of semantic content.

Perhaps the most famous example of a grammatically correct sentence thought by many to be semantically meaningless is Noam Chomsky's Colorless green ideas sleep furiously, which can be diagrammed into a phrase tree as below:

Where σ represents a grammatical sentence.

This phrase tree can also be represented with the following Lisp S-expression:

  ((NP (ADJ colorless) (NP (ADJ green) (N ideas)))

   (VP (V sleep) (ADV furiously)))

There are, however, difficulties with this type of structure. For example, normal TGG rules state "VP --> VP|NP_o" with NP_o being the "object" of the verb (and "|" replacing "+" to indicate that phrasal sequence is not relevant to the current discussion). This presents no difficulty with, for example, languages with SOV or SVO typology[?], but this does not account very well for the few OSV and VSO languages that exist.

TGG and phrase-structure rules have been largely abandoned by structural linguistics[?] for this reason, although it still has useful applications in language-specific research. PSRs also continue to be useful in the study of children's language acquisition, the study of teaching foreign languages, and the field of Universal Grammar.