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Very rarely, the nucleus of a single syllable may contain three vowel sounds that quickly glide together; these compound sounds are known as triphthongs.
There are three triphthongs that are generally agreed upon in American English: /aʊə/ (“ah-oo-uh”), /aɪə/ (“ah-ih-uh”), and /jʊə/ (“ee-oo-uh”). These always come before an R sound in a word.
triph·thong(trĭf′thông′, -thŏng′, trĭp′-)
A compound vowel sound resulting from the succession of three simple ones and functioning as a unit.
triph·thon′gal (-thông′əl, -thŏng′əl) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
1. (Phonetics & Phonology) a composite vowel sound during the articulation of which the vocal organs move from one position through a second, ending in a third
2. (Phonetics & Phonology) a trigraph representing a composite vowel sound such as this
[C16: via New Latin from Medieval Greek triphthongos, from tri- + phthongos sound; compare diphthong]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
triph•thong(ˈtrɪf θɔŋ, -θɒŋ, ˈtrɪp-)
1. a monosyllabic speech-sound sequence made up of three differing vowel qualities, as in some pronunciations of our.
[1590–1600; < New Latin triphthongus < Medieval Greek tríphthongos with three vowels =tri- tri- + phthóngos voice, sound]
triph•thong′al (-gəl) adj.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.