[2431.] The vowel is short in:

[2432.] (a.) Monosyllables ending in b, d, m, and t: as, ab, ad, dum, dat.

[2433.] (b.) The indefinite qua, N. and Ac.; the enclitics -que (rarely -quē), -ne, -ve, -ce; and in the words cor, fel, mel; os, bone; ac, vir, is, pol, quis (N.); fac, fer, per, ter; an, bis, in, cis; nec, vel. N. hīc is rarely short ([664]). For the quantity of es, see [747].

[Polysyllables.]

Penults.

[2434.] Disyllabic perfects and perfect participles have the vowel of the penult long when it stands before a single consonant: as,

vēnī, vīdī, vīcī ([862]); fōvī ([864]), fōtus ([917]).

Exceptions.

[2435.] (a.) Nine perfects have the penult short ([859-861]):

bibī, -fidī