[*] 10. Vowels are either long or short by nature, and are pronounced accordingly (§ 8). [*] a. A vowel before another vowel or h is short: as in vĭa, nĭhil. [*] b. A diphthong is long: as in aedēs , foedus . So, also, a vowel derived from a diphthong: as in exclūdō (from † ex-claudō ). [*] c. A vowel formed by contraction is long: as in nīl (from nihil ). [*] d. A vowel before ns, nf, gn, is long: as in cōnstāns , īnferō , māgnus .
[*] Note.--But the quantity of the vowel before gn is not certain in all cases.[*] e. A vowel before nd, nt, is regularly short: as in amandus , amant . In this book all vowels known to be long are marked (ā, ē, etc.), and short vowels are left unmarked (a, e, etc.). Vowels marked with both signs at once (ā˘, ē˘, etc.) occur sometimes as long and sometimes as short.
[*] Note.--The Romans sometimes marked vowel length by a stroke above the letter (called an apex), as, Á; and sometimes the vowel was doubled to indicate length. An I made higher than the other letters was occasionally used for ī. But none of these devices came into general use