Eades, Domenyk; Hajek, John, Gayo, Journal of the International Phonetic Association, 2006, 36 (1): 107–115, doi:10.1017/S0025100306002416
Gordon, Matthew J., New York, Philadelphia, and other northern cities: phonology, Schneider, Edgar W.; Burridge, Kate; Kortmann, Bernd; Mesthrie, Rajend; Upton, Clive (編), A handbook of varieties of English, 1: Phonology, Mouton de Gruyter: 282–299, 2004, ISBN 3-11-017532-0
Ternes, Elmer; Vladimirova-Buhtz, Tatjana, Bulgarian, Handbook of the International Phonetic Association, Cambridge University Press: 55–57, 1999, ISBN 0-521-63751-1
Urua, Eno-Abasi E., Ibibio, Journal of the International Phonetic Association, 2004, 34 (1): 105–109, doi:10.1017/S0025100304001550