One of the direct followers of the GPSG was called Head-Driven Phrase Structure Grammar (HPSG). In addition to the advanced traits of the GPSG, it has introduced and intensively used the notion of head. In most of the constituents, one of the sub-constituents (called daughters in HPSG) is considered as the principal, or its head (called also head daughter). For example, in the rule:
S ® NP(Pers, Num) HVP(Pers, Num),
the VP constituent (i.e., the syntactic predicate of a sentence with all connected words) is marked as the head of the whole sentence, which is indicated by the symbol H. Another example: in the rule:
NP (Gen, Num) ® D (Gen, Num) HN (Gen, Num),
the noun is marked as the head of the whole noun phrase NP.
According to one of the special principles introduced in HPSG, namely the head principle, the main features of the head are inherited in some way by the mother (enclosing) constituent (the left-side part of the rule).
In the previous examples, the features of the predicate determine features of the whole sentence, and the features of the noun determine the corresponding features of the whole noun phrase. Such formalism permits to easier specify the syntactic structure of sentences and thus facilitates syntactic analysis (parsing).
As it was already said, the interpretation in early generative grammars was always of syntactic nature. For semantic interpretation (“understanding”), additional theoretical means were introduced, which were somewhat alien to the earlier generative structure mainstream. By contrast, each word in the HPSG dictionary is supplied with semantic information that permits to combine meanings of separate words into a joint coherent semantic structure. The novel rules of the word combining gave a more adequate method of construing the semantic networks. Meantime, Chomskian idea of transformations was definitely abandoned by this approach.