^ Mallory, J. P. with Adams, D. Q. (2006) The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World (Oxford Linguistics), New York: Oxford University Press, →ISBN, page 255
^ Sihler, Andrew L. (1995) New Comparative Grammar of Greek and Latin, Oxford, New York: Oxford University Press, →ISBN
↑ 6.06.1Guus Kroonen (2013) “*keusan-”, in Alexander Lubotsky, editor, Etymological Dictionary of Proto-Germanic (Leiden Indo-European Etymological Dictionary Series; 11)[2], Leiden, Boston: Brill, →ISBN, page 286
↑ 7.07.1Guus Kroonen (2013) “*kusti-”, in Alexander Lubotsky, editor, Etymological Dictionary of Proto-Germanic (Leiden Indo-European Etymological Dictionary Series; 11)[3], Leiden, Boston: Brill, →ISBN, page 313
^ Guus Kroonen (2013) “*kustu-”, in Alexander Lubotsky, editor, Etymological Dictionary of Proto-Germanic (Leiden Indo-European Etymological Dictionary Series; 11)[4], Leiden, Boston: Brill, →ISBN, page 313