IVXLCDM1510501005001000
β’ A smaller symbol before a larger one subtracts: IV = 4, IX = 9, XL = 40, XC = 90, CD = 400, CM = 900.
β’ Only I, X and C may be subtracted, and only from the next two larger symbols.
β’ No symbol repeats more than three times in a row (IIII is non-standard, though clock faces use it).
β’ A bar over a numeral multiplies it by 1,000: VΜ
= 5,000, XΜ
= 10,000.
β’ There is no zero. The Romans used nulla (βnoneβ) in words.