the Roman letters used as numerals until the 10th cent. : in Roman numerals I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, and M = 1,000 Other numbers are formed from these by adding or subtracting: the value of a symbol following another of the same or greater value is added (e.g., III = 3, XV = 15); the value of a symbol preceding one of greater value is subtracted (e.g., IX = 9); and the value of a symbol standing between two of greater value is subtracted from that of the second, the remainder being added to that of the first (e.g., XIX = 19): Roman numerals are commonly written in capitals, though they may be written in lowercase letters, as in numbering subdivisions (e.g., Act IV, scene iii) A bar over a letter indicates multiplication by 1,000 (e.g.,  = 5,000)