^(with *h₁)Ringe, Donald (2006)From Proto-Indo-European to Proto-Germanic (A Linguistic History of English; 1)[1], Oxford: Oxford University Press,→ISBN
^(with h₂ or h₃)De Vaan, Michiel (2008)Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill,→ISBN