Phrase structure rules

Phrase-structure rules were used in early transformational-generative grammars (TGG) to describe a given language's syntax. This was 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. Phrase structure rules were not an invention of TGG; rather, early TGG's defining characteristics were those systems which it had in addition to phrase structure rules (the most obvious example being transformations). The page transformational grammar gives an overview of the development of TGG.

PSRs must, in most theories, account for the following characteristics:

1. All languages combine nouns (N) and verbs (V) to express ideas about the universe.
2. All languages have rules determining how these are combined into meaningful units.
3. All languages have recursion, i.e. rules 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 might be described using phrase structure rules as following:
      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 has usually attempted to set down or categorize all the rules of a language that lead to grammatical utterances, regardless of semantic content. However, the border between syntax and semantics is not easy to define, and there have been many shifts of perspective in this regard throughout the development of grammatical theories in general.

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, early TGG rules stated "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.

Phrase-structure rules have been largely abandoned by structural linguistics for this reason, and they have also been absent from transformational theories for some time, though not for the same reason. PSRs continue to be useful in the study of children's language acquisition, the study of teaching foreign languages, and the field of Universal Grammar.