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 (dĭf′thông′, -thŏng′, dĭp′-)
A complex speech sound or glide that begins with one vowel and gradually changes to another vowel within the same syllable, as (oi) in boil or (ī) in fine.

[Middle English diptonge, from Old French diptongue, from Late Latin diphthongus, from Greek diphthongos : di-, two; see di-1 + phthongos, sound.]

diph·thon′gal 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.
References in periodicals archive ?
Here, Stevens seems to be saying that the original pronunciation of Tummeor was [te'mi:] or [te'mi:e], with the centring diphthongal glide represented by the final -r in his written form.
short, long and diphthongal); b) there are hypothetically as many phonemic types as phonetically discernible.
Concretely, the author addresses the phonemes /e/ and /o/ as more or less diphthongal monophthongs.
A note on DRESS comments on its 'upgliding', diphthongal realisation before /d/ and /n/ (generally dental in Shetland).
The diphthongal forms, first recorded, according to him, in the Ancrene Riwle, were rare in other 13th century texts.
And Tosca sings L'ardente amante mia, with the diphthongal adjective set to two dotted quarter notes.
This, in turn, is counterpoised by the sudden diphthongal echo of "mine" in "thine." Rather than being a bequest, the "such" seems still safeguarded by the "mine"--it will, hence, only be thine "when with me."
For GOOSE, which has both monophthongal and diphthongal variants, we find that the younger speaker's pronunciations are much more fronted than the older speaker's, overlapping with FLEECE to some extent.
The original (lost) Visigothic language may have had both diphthongal and monophthongal sounds in opposition to each other in the relevant lexemes, but we do not know how they were spelt in Visigothic manuscripts.
New Yawk Tawk also features a diphthongal /aw/ sound that in heavy Nooyawkese sounds almost disyllabic.
1988b "'Historische Grammatik and Middle English diphthongal systems", in: Dieter Kastovsky - Gero Bauer (eds.), 419-433.
The old diphthongs are a salient dialect feature that is characteristic of dialects spoken in remote parts of the Swedish language area, and diphthongal realizations (or secondary diphthongs) of the long vowel phonemes are, in Finland, confined to the dialects of a small area north and south of Kristinestad.