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. ”
