Halliday & Matthiessen (1999: 517):
As for the elements that make up a process configuration, we have seen that the foundation of the grammar’s theory of experience was laid down in the simplest terms in the evolution of word classes: verbs, nouns and others. But the constituents of a clause are not, in fact, verbs and nouns; they are more complex expressions that have expanded from verbs and nouns, which we call “verbal groups” and “nominal groups”. At this point, we find considerable difference between the two, in the kind of expansion that they engender. The formal patterns vary, as always, among different languages; but the underlying principles seem to be fairly constant.