(1991) Grammaticalization: A conceptual framework. Chicago: University of Chicago Press.
Lichtenberk, E.
(1991) On the gradualness of grammaticalization. In E. C. Traugott & B. Heine (Eds.), Approaches to grammaticalization. Vol. 11 (pp. 37–80). Amsterdam: John Benjamins.
Mufwene, S. S.
(1996) Creolization and grammaticization: What creolistics could contribute to research on grammaticization. In P. Baker & A. Syea (Eds.), Changing meaning, changing functions (pp. 5–28). London: Westminster University Press.