Viëtor 1899, p.3. (de) Wilhelm Viëtor, Deutsches Lesebuch in Lautschrift (zugleich in der amtlichen Schreibung), vol.1, Leipzig, B.G. Teubner, (lire en ligne)
Pitman 1845. (en) Isaac Pitman, «Completion of the Phonotypic Alphabet», The Phonotypic Journal, 5 Nelson Place, Bath, Phonographic Institution, vol.4, no42, , p.105-106 (lire en ligne)
IPA 1993, p.33. (en) International Phonetic Association, «Council actions on revisions of the IPA», Journal of the International Phonetic Association, vol.23, no1, , p.32-34 (DOI10.1017/S002510030000476X)