Institutio Oratoria

Similarly the letter Q, which is superfluous and useless save for the purpose of attaching to itself the vowels by which it is followed, results in the formation of harsh syllables, as, for example, when we write equos and aequum, more especially since these two vowels together produce a sound for which Greek has no equivalent and which cannot therefore be expressed in Greek letters. [*]( The sound of Q in itself does not differ from C. It would therefore be useless, save as an indication that U and another vowel are to follow. The U in this combination following Q was, as Donatus later pointed out, neither a vowel nor a consonant, i.e. it was something between U and V. )