^ Ringe, Donald (2006) From Proto-Indo-European to Proto-Germanic (A Linguistic History of English; 1)[1], Oxford: Oxford University Press, →ISBN
^ Guus Kroonen (2013) “*at”, in Alexander Lubotsky, editor, Etymological Dictionary of Proto-Germanic (Leiden Indo-European Etymological Dictionary Series; 11)[2], Leiden, Boston: Brill, →ISBN, page 39
^ De Vaan, Michiel (2008) “ad”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page 24
^ Garnier, Romain (2014) “Nouvelles réflexions sur l’effet-Kortlandt”, in Glotta (in French), volume 90, Vandenhoeck & Ruprecht, pages 140-160
^ Ačaṙean, Hračʻeay (1979) “ց”, in Hayerēn armatakan baṙaran [Armenian Etymological Dictionary] (in Armenian), 2nd edition, a reprint of the original 1926–1935 seven-volume edition, volume IV, Yerevan: University Press, page 446a
Andrew Byrd, The Indo-European Syllable (2015), page 117: the reader should note that monosyllabic lengthening did not occur in PGmc. *hʷát "what" (< PIE *kʷód; not PGmc. ˟hʷṓt) or *át "at" (< PIE *ád; not PGmc. ˟ṓt), two words of similar shape (Ringe 2006:98).
