Ladefoged, Peter; Maddieson, Ian (1996). The Sounds of the World's Languages (bằng tiếng Anh). Oxford: Blackwell. ISBN0-631-19814-8.
McDorman, Richard E. (1999). Labial Instability in Sound Change: Explanations for the Loss of /p/. Chicago: Organizational Knowledge Press. ISBN 0-967-25370-5.