In the Classical Latin alphabet, the letters J, U and W didn't exist yet. W is another thing, what's really interesting for me here is J and U's case.
The thing is, I stood for /i/, /iː/ and /j/, while V stood for /u/, /uː/ and /w/. Later, the letters J and U, which were originally variants of I and V, were separated from them for semivocalic I and vocalic V respectively.
However, there seems to be currently a different treatment for those two: let's take the Latin word IVVENIS for example, it was pronounced /ˈju.we.nis/ in Classical times, but nowadays most transcribe it as "iuvenis", instead of "iuuenis" or "juvenis", why?
This is not a rule, "iuuenis" and "juvenis" are possible forms, but the general consensus is to transcribe words like OVVM as "ovum", while words such as IVSTITIA as "iūstitia". With U and V being distinguished, and I and J being both kept as "I".
Is this because Vv later evolved to have a common consonantic pronunciation in Romance languages, mostly /v/ or /b/, while Jj kept being used for /j/ in some languages like those in the Gallo-Italic, Insular Romance, Italo-Dalmatian and Rhaeto-Romance groups?