To my knowledge every vowel is either long, short or belongs to a diphthong, there are no vowels which are medium in length. If a vowel belongs to a diphthong, it seems that modern writers will mark the second vowel either long or short. Every once in a while a vowel will have both the macron dash, meaning long and the oblique c meaning short on the same vowel, which I guess means that it is not known what the vowel's quantity was or both were possible. It follows that if a vowel is not marked long or short then it must be followed by a vowel with which it forms a diphthong. I've never seen anyone spell this out as a rule, but as far as I can see this would make the most sense. However, if this is the correct rule I've noticed a lot of writers apply this rule inconsistently. Some vowels will be neither marked long nor short and they clearly do not belong to a diphthong as they are surrounded by consonants. For example, at fatus, as you can see sometimes vowels are marked long or short but quite often neither are marked. Does this mean that the vowel is short if it is not marked? It seems that the writer wanted to mark every vowel short with an oblique c but got lazy and gave up. I realize that that site is very unreliable but it's much easier to use than Collatinus. At Collatinus it seems that the authors have taken the trouble to mark every vowel long or short but there are some cases I'm not sure about. collatinus I cannot link directly to the verb conjugation but if you conjugate "edo" you'll see that the Indicatif perfectum futur antérieur 1st person is ēdĕrō̆. I'm not exactly sure what the vowel quantity of 'o' is. Also, if you conjugate 'aequore', you'll see that e is not marked long or short, I'm guessing because it is a diphthong but this supposition is not consistent with their not marking 'u'. As far as I'm aware there are 7 diphthongs in Latin, ei, eu, oe, ae, au, ou and ui. uo is not a diphthong. So why did they not mark u either long or short? If they do believe it is a diphthong then why did they not put the mark on the first vowel?
The same is also true of the Lewis dictionary 1879. If you look at the entry for 'insula' you find "insŭla". What, does he think it is obvious what the quantity of 'i' and 'a' is but for 'u' he decided to make it entirely clear? This is very annoying.
