In linguistics, the term phrase is used to refer to a group of words acting together as a unit. Groups of words can form a unit with respect to different properties: their meaning (PHRASAL COLLOCATIONS), their intonational contour (INTONATIONAL PHRASES) or their structural properties (SYNTACTIC PHRASES). The different types of phrases can be orthogonal to each other; for example a phrasal collocation need not coincide with an intonational phrase, or with a syntactic phrase. Phrase structure grammars are models of constituent structure that rely on syntactic phrases as primary building blocks.