中後圓唇元音2010 [2018-04-08], (原始内容 (PDF)存档于2018-11-01) Árnason, Kristján, The Phonology of Icelandic and Faroese, Oxford University Press, 2011, ISBN 978-0-19-922931-4
中前不圆唇元音Gordeeva, Olga B.; Matthews, Benjamin, Acquisition of Scottish English Phonology: an overview, Edinburgh: QMU Speech Science Research Centre Working Papers