• Webster's Revised Unabridged Dictionary
    • a Triphthongal Of or pertaining to a triphthong; consisting of three vowel sounds pronounced together in a single syllable.
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Century Dictionary and Cyclopedia
    • triphthongal Pertaining to a triphthong; consisting of a triphthong.
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In literature:

A TRIPHTHONG is produced by running together three vowel sounds in the same syllable.
"Orthography" by Elmer W. Cavins
The Serbian language being phonetic does not employ double consonants, diphthongs or triphthongs.
"Hero Tales and Legends of the Serbians" by Woislav M. Petrovitch