In a general, pre-theoretical sense, this term is often used in linguistics and phonetics to refer to any entity which constitutes the focus of an enquiry. In Hallidayan linguistics, however, the term has a special status, referring to one of the four main categories recognized by that theory (the others being structure, class and system). The unit is the stretch of language that carries grammatical patterns, and within which grammatical choices are made. For example, the unit sentence consists of one or more instances of the unit clause, and so on. In some grammatical descriptions, the term unit noun is preferred to countable noun.