a vowel sound modified by umlaut.
Are There Any Words That Use “W” As A Vowel?A, E, I, O, U, and sometimes Y ... and W? Yes, the letter W can behave as a vowel. It's time to level up your Scrabble game, people. And, to all our grade-school peeps out there, get ready to knock the socks off your Spelling teacher.
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British Dictionary definitions for vowel mutation
another name for umlaut
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