Head-driven Phrase Structure Grammar combines formal rigidity with expressiveness. This makes the framework attractive for empirically working linguists as well as for computational linguists and researchers interested in the formal complexity of natural languages.
HPSG has its roots in Phrase Structure Grammars, and makes use of traditional notions such as constituent structures, syntactic categories, selection. To incode all these in a uniform formalism, HPSG uses a formal device, called feature structures as the representations of linguistic objects. Feature structures are a flexible device that allows to encode all linguistic information.
The following pages give you information on the formal notions and terminology that is used in HPSG.