a. A vowel before another vowel or h is short: as in via, nihil.
b. A diphthong is long: as in áédes, fóédus. So, also, a vowel derived from a diphthong, as in exclúdó (from ¥ex-claudó).
c. A vowel formed by contraction is long: as in níl (from nihil).
d. A vowel before ns, nf, gn, is long: as in cónstáns, ínferó, mágnus.
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 (á, é, etc.), and short vowels are left unmarked (a, e, etc.). Vowels marked with both signs at once (áád0.3ex, ééd0.3ex, 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 í. But none of these devices came into general use.