Ladefoged, Peter. (1993). A course in phonetics (3rd ed.). Fort Worth: Harcourt College Publishers.
Roach, Peter (2004), "British English: Received Pronunciation", Journal of the International Phonetic Association34 (2): 239-245
Thomas, Erik R. (2001), An acoustic analysis of vowel variation in New World English, Publication of the American Dialect Society, 85, Duke University Press for the American Dialect Society, ISSN0002-8207