[Ars Gram. Mar. Vict. de orthographia et de metrica ratione, I. vi. 6.] A littera rictu patulo, suspensa neque impressa dentibus lingua, enuntiatur.
E is uttered with the mouth less widely open, and the lips drawn back and inward:
[Id. ib. vi. 7.] E quae sequitur, de represso modice rictu oris, reductisque introrsum labiis, effertur.
I will voice itself with the mouth half closed and the teeth gently pressed by the tongue:
[Id. ib. vi. 8.] I semicluso ore, impressisque sensim lingua dentibus, vocem dabit.
O (long) will give the “tragic sound” through rounded opening, with lips protruded, the tongue pendulous in the roof of the mouth:
[Id. ib. vi. 9.] O longum autem, protrusis labiis, rictu tereti, lingua arcu oris pendula, sonum tragicum dabit.
U is uttered with the lips protruding and approaching each other, like the Greek ου:
[Id. ib. vi. 10.] U litteram quotiens enuntiamus, productis et coeuntibus labris efferemus . . . quam nisi per ου conjunctam Graeci scribere ac pronuntiare non possunt.
Of these five vowels the grammarians say that three (a, i, u) do not change their quality with their quantity: