ita dixerim, vocis vicinitas quaedam est, linguae sublatione ac
positione distinguuntur. Nam cum summos atque imos conjunctim dentes
suprema sui parte pulsaverit D litteram exprimit. Quotiens autem
sublimata partem, qua superis dentibus est origo, contigerit T sonare
vocis explicabit.

But when certain words in common use ending in D were followed by words
beginning with a consonant, the sound of the D was sharpened to T; and
indeed the word was often, especially by the earlier writers, written
with T, as, for instance, _set_, _haut_, _aput_:

[Mar. Vict. I. iii. 50.] D tamen litteram conservat si sequens verbum
incipiat a vocali; ut _haud aliter muros_; et _haud equidem_. At cum
verbum a consonante incipit, D perdit, ut _haut dudum_, et _haut
multum_, et _haut placitura refert_, et inducit T.

F is pronounced as in English except that it should be brought out more
forcibly, with more breath.

[Keil. v. VI. p. 32.] F litteram imum labium superis imprimentibus
dentibus, reflexa ad palati fastigium lingua, leni spiramine proferemus.

Marius Victorinus says that F was used in Latin words as PH in foreign.

Diomedes (of the fourth century) says the same:

[Diom. Keil. v. I. p. 427.] Id hoc scire debemus quod F littera tum
scribitur cum Latina dictio scribitur, ut _felix_. Nam si peregrina
fuerit, P et H scribimus, ut _Phoebus_, _Phaethon_.