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cardinal vowels

British  

plural noun

  1. a set of theoretical vowel sounds, based on the shape of the mouth needed to articulate them, that can be used to classify the vowel sounds of any speaker in any language

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012