20. Latin had the following vowels, which might be long or short: a, e,
i, o, u. The diphthongs, æ, œ, au, eu, ui, were always long: æ and œ,
however, were simplified into monophthongs, mainly in the Republican
epoch, _æ_ being sounded ę̄, _œ_ probably ẹ̄; au retained (save in some
popular dialects) its old pronunciation; eu did not occur in any word
that survived; ui, in _cui_, _illui_, in Vulgar Latin, was accented
_úi_ (as in _fui_). The simple vowels, except a, were, doubtless from
early times, slightly different in quality according to their quantity,
the long vowels being sounded close, the short open: ẹ̄, ị̄, ọ̄, ụ̄;
ę̆, į̆, ǫ̆, ų̆.
21. Between the 1st and the 7th century of our era, the Classic Latin
quantity died out: it had apparently disappeared from unstressed
vowels as early as the 4th century, from stressed by the 6th. It left
its traces, however, as we have seen, upon accentuation (§ 16), and
also upon vowel quality, the originally long and short remaining
differentiated in sound, if they were accented. Of the unaccented
vowels, only i shows sure signs of such a differentiation, and even for
i the distinction is evident only in a final syllable: _vēnī_ _vēnĭt_ ＞
vẹnị vẹnįt.