How one grammatical unit relates to another is its function. For example, in the clause ‘the meeting started late’, ‘the meeting’ is the subject. This describes its relation to ta verb (and a clause). However, in the clause ‘they started the meeting late’, the same words (‘the meeting’) stand in a different relation to the verb: they are functioning as its object.

A class is a set of grammatical units that are alike in a language system, such as noun, verb, adjective and corresponding groups/phrases: noun group/phrase, verb group/phrase, adjective group/phrase.’ For example, to say that ‘the meeting’ is a noun group/phrase is to say that it is the same kind of unit as ‘a book’, ‘that car’, ‘my uncle’.