Halliday & Matthiessen (2014: 68):
Grammaticalisation is not dependent on how the categories are realised. They may be realised in a variety of ways:
- a change in the form, articulatory or prosodic, of some word or words;
- an addition of some element, to a word, a group or a clause;
- a change in the order of words, groups, or clauses.
The realisation may not be the same for all categories or in all environments; but it will be systematic in some way in the majority of cases, enough to establish and maintain the proportionality – with only a minority of ‘exceptions’ (which are likely to include some of the more frequent items).