Ian., Maddieson (1984). Patterns of sounds. Cambridge [Cambridgeshire]: Cambridge University Press. ISBN9780521113267. OCLC10724704.
L., Bybee, Joan (1994). The evolution of grammar : tense, aspect, and modality in the languages of the world. Perkins, Revere D. (Revere Dale), Pagliuca, William. Chicago: University of Chicago Press. ISBN0226086631. OCLC29387125.{{cite book}}: CS1 maint: multiple names: authors list (link)
Givón, Talmy (2001). Syntax : an introduction. Volume 1 (Rev. ed.). Amsterdam: J. Benjamins. ISBN1588110656. OCLC70727915.