^ Sihler, Andrew L. (1995) New Comparative Grammar of Greek and Latin, Oxford, New York: Oxford University Press, →ISBN, page 418
^ Ringe, Donald (2006) From Proto-Indo-European to Proto-Germanic (A Linguistic History of English; 1)[1], Oxford: Oxford University Press, →ISBN, page 206
^ Beekes, Robert S. P. (2011) Comparative Indo-European Linguistics: An Introduction, 2nd edition, revised and corrected by Michiel de Vaan, Amsterdam, Philadelphia: John Benjamins Publishing Company, page 238
