Roman numeral, any of the symbols used in a system of numerical notation based on the ancient Roman system. The symbols are I, V, X, L, C, D, and M, standing respectively for 1, 5, 10, 50, 100, 500, and 1,000 in the Hindu-Arabic numeral system.
A symbol placed after another of equal or greater value adds its value; e.g., II = 2 and LX = 60.
A symbol placed before one of greater value subtracts its value; e.g., IV = 4, XL = 40, and CD = 400. A bar placed over a number multiplies its value by 1,000.
Studies of inscriptions left by the Etruscans, who ruled Italy before the Romans, show that the Romans adopted the Etruscan numerical system beginning in the 5th century bce but with the distinct difference that the Etruscans read their numbers from right to left while the Romans read theirs from left to right. L and D for 50 and 500, respectively, emerged in the Late Roman Republic, and M did not come to mean 1,000 until the Middle Ages.