次閉元音Basbøll, Hans, The Phonology of Danish, 2005, ISBN 0-203-97876-5 Doke, Clement Martyn; Mofokeng, S. Machabe, Textbook of Southern Sotho Grammar 3rd, Cape
閉前不圓唇元音Europa, 1995 [2023-06-15], ISBN 9783929075083, (原始内容存档于2023-04-21) Doke, Clement Martyn; Mofokeng, S. Machabe, Textbook of Southern Sotho Grammar 3rd, Cape