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:
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|NPo" with NPo 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.