Halliday & Matthiessen (1999: 124-5):
As we noted in Chapter 1, Section 1.5, semantics and grammar evolve together (in all three senses of semohistory we discussed). In the present context that means that sequences and clause complexes evolve together. The basic principle is that a sequence is realised by a clause complex. But the two may become dissociated from one another.
(i) On the one hand, a sequence may extend beyond a single clause complex. That is, the general potential is simultaneously semantic and grammatical; but in the creation of this particular text, this potential may be taken up semantically to create a sequence that is more extensive than the clause complexes realising it. Here is a very simple example of a sequence which is realised by two clause complexes:
Prepare and cook the potatoes in sea-salted water to which has been added lemon juice. Drain and serve with melted butter, a few shreds of lemon rind, a squeeze of lemon juice and chopped parsley.
The sequence has been grouped into two clause complexes in the grammar. However, this grouping is by no means arbitrary; it serves to indicate the two major phases of the preparation of new potatoes with lemon. In other words, once sequence and clause complex have become partly disassociated so that one sequence does not automatically imply one clause complex, the decision how to associate them in realisation becomes a meaningful, significant choice. (The significance may vary from one register to another, but the principle that the choice is meaningful is quite general.)
