The Roman Empire did not use the same number system that we use today.
They created their own set of numerals. Roman numerals were used only to record numbers. Their great strength is that they are difficult to falsify and so they remained in use for recording numbers for business and administration long after decimal notation was in use for calculation. People throughout Europe used Roman numerals until A.D. 1500. Roman number symbols did not evolve continuously and smoothly any more than did the Roman number words. The Roman numerals are the "sisters" of the Latin spoken numbers. Like the words the numerals were not invented suddenly by one person, but were evolved gradually by the people who used them.
Bigger numbers were sometimes made with a bar over the letter, to indicate "multiply by 1000".
The numbers 4 and 9 can be written two ways:
IV=4, I+I+I+I=4
VIIII=9, IX=9
Here we see that I, before V, is -1, not +1. Also IX = 9, XC = 90, CD = 400, etc. A lesser number before a greater number is negative. Things like IIX or IC are not allowed. We only can have one consecutive negative symbol. And the negative I can only occur before a V or an X. There is no Roman numeral zero.
