From the Things I Did Not Know files, courtesy of Nvmeri Innvmeri:
The Romans had no negative numbers, and could express only a limited range of fractions with numeric symbols, principally the twelfths from 1/12 to 11/12. (The American pound has 16 ounces, but the Roman libra was divided into 12 unciae.) That means that while any properly-formed Roman number can be expressed in Arabic numbers, the converse is not true.
Since Wilson Pickett has been on my mind of late, I asked for a Roman equivalent of 99½, which comes back XCIX S; apparently one through five twelfths are represented by horizontal lines ordered in pairs, and seven through eleven by S plus those same lines, so 999/12 would be XCIX S= – or something like that.