The mechanism of the change illustrates a process referred to as grammaticalization. Meillet [Meillet, A. (1912/1958). “L’évolution des formes grammaticales”, Scientia 12. [Also in Linguistique historique et linguistique générale. Paris: Campion, pp. 130-149].] identifies this as one major source of grammatical change in language (the other is analogy), in which autonomous words become grammatical markers.
- Romaine (1988), a pag.133