Diphthong

Definition and stakes

Peter Giles,  1911 Encyclopædia Britannica (1911)

“ Before a following vowel the diphthong was divided between the two syllables, the ι or υ forming a consonant at the beginning of the second syllable, which ultimately disappeared. Thus from a root dheu- “run” comes a verb θέω for θε–ϝω, from an earlier *θευ–ω. The corresponding adjective is θοός “swift,” for θο–ϝο–ς, from an earlier *θου–ο–ς. The only dialect which kept the whole diphthong in one syllable was Aeolic. The long diphthongs, except at the ends of words, were shortened in Attic. Some of these appear merely as long vowels, having lost their second element in the proethnic period. ”
Source: Wikisource

Various,  The Continental Monthly, Vol 6…

“ This Diphthong IU (or yiu) denotes Conjunction, Conjuncture, Event (the two ends meeting) ; and also Coupling or Unition; a central point between extremes.
The next and the most important of the Diphthongs (except AU) is AI, compounded of the third (A) and the first (I) of the simple Vowel-Sounds. It is pronounced very nearly like the English long I, as in pine, fine, etc., which is not a simple Vowel; but is compounded of the two simple Vowels above mentioned (A and I, ahee) in a very close union with each other
”
Source: Gutenberg

A Spanish Anthology

“ If the first of two contiguous vowels before the accent is strong and the second is weak, they naturally form a diphthong; vaivén, peinado. They are regularly in distinct syllables, however, when the first vowel (except a) is part of a prefix, as in preinserto, prohijar, rehusado (prefixes pre-, pro-, re-) . Nevertheless, synæresis is here permitted, and the diphthong is normal where a is the vowel of the prefix, as in airado, ahumado.
(3) Usage varies when of two contiguous vowels before the accent the first is weak and the second strong. The derivative follows the simplex
”
Source: Gutenberg

Get perspective with Kwize: daily news enlightened by great literature