This question originates from this thread upon suggestion of Joonas Ilmavirta.

Q. How do we know all the vowel lengths in Latin?

It would be of interest to me if we manage to collect a list with all the rules determining the length of a vowel (hence also correct pronunciation) of a word. Quoting Joonas, "books rarely explain how we know things, they just give the things as facts".

As an example, the rule "Vocalis ante vocalem brevis est" (mentioned py P. Majer in the aformentioned topic) was unknown to me (together with the exception pointed out by Joonas: "There are rare cases of long vowels before vowels, like fio").

  • It’s not quite clear to me: do you want to know the rules per se (i.e. descriptive observations) or do you want to understand what evidence these rules are based on (i.e. some kind of theoretical explanation)?
    – Alex B.
    Commented Nov 11, 2019 at 15:41
  • Well, to begin with, I would be satisfied in having a list with all (?) the rules used to determine the quantity of a vowel (like the one mentioned in the post). Of course it would be also interesting to have a theoretical explanation on why that particular rule holds true, but maybe better to postpone that to a successive moment (say, something like statement vs proof of a theorem: let us confine ourselves first to the statements). Is it more clear? Thanks for your comment, btw.
    – Romeo
    Commented Nov 11, 2019 at 16:47
  • 1
    I understood your question in the second sense mentioned by Alex B. As to the rules: these are usually listed in any grammar text, starting with the distinction between length "by position" and "by nature". E.g a vowel followed by a couple of consonants is long, although with a short list of exceptions, etc. Note also that declension and conjugation terminations do have their lengths. Finally, in any good dictionary you may also find the lengths of each word "by nature" Commented Nov 11, 2019 at 17:12
  • @PietroMajer Thanks a lot for your comment and answer. For some reason, I was conscious of the distinction "long by position" vs "long by nature" in Greek, but I was not aware of that in Latin. Just for completeness, could you please mention a complete grammar book containing such a list? I cannot check now the one I used at school (I have it at home and I am living abroad) but I am pretty sure it was not complete in this respect). I am going to comment your answer asap. Meanwhile, thanks again!
    – Romeo
    Commented Nov 12, 2019 at 11:11

1 Answer 1


I'd say the main source of our knowledge is poetry. This was true for the Greek grammarians, who invented the notion itself of vowel length, and created prosody as a scientific linguistic theory, aimed to learn how to read and compose correctly poetry (and, later on, rhetoric too). So they classified the phenomena and inferred abstract rules, together with their exceptions. When the Greek civilization reached Rome, Latin poets adopted this theory, and the various Greek verses, starting with Ennius; rules of Latin prosody are not always the same of the Greek, of course. And this is also how we individually learn: when we have read e.g. some hundreds of hexameters, we naturally learn the quantities of many words, and say, the difference between pŏpŭlus (m, people) and pōpŭlus (f, poplar tree), and we naturally learn a number of empiric rules.

To make a simple example: if an hexameter has 17 syllables, the maximum number allowed, it must be composed by 5 dactyls ( – ⏑ ⏑ ) plus a closing foot, which is either a trochee ( – – ) or spondee ( – ⏑ ). Similarly, if it has 12 syllables, the minimum allowed number, it must be done by 5 trochees plus either one more trochee or a spondee. In either case, all lengths are determined (but the last one, to be precise).

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