The Romans used a decimal system for whole numbers, reflecting how they counted in Latin and they used a duodecimal system for fractions, because the divisibility of twelve makes it easier to handle the common fractions of 1/3 and 1/4 than does a system based on ten.
On coins, many of which had values that were duodecimal fractions of the unit as, they used a tally-like notational system based on twelfths and halves. A dot indicated an uncia "twelfth", the source of the English words inch and ounce; dots were repeated for fractions up to five twelfths.
Six twelfths was abbreviated as the letter S for semis "half". Uncia dots were added to S for fractions from seven to eleven twelfths, just as tallies were added to V for whole numbers from six to nine. Now, you can be more confused about the roman numeral system.
