Halliday & Matthiessen (2014: 74):
A class is a set of items that are in some respect alike. The most familiar, in our traditional grammar, are classes of words: verb, noun, adjective, adverb, pronoun, preposition, conjunction (and sometimes also interjection), in the usual list. But every unit can be classified: there are classes of group and phrase, classes of clause, and, at the other end of the rank scale, classes of morpheme.
