Roman numerals are a system of numerical notation used by the Romans. The basic letters are I, V, X, L, C, D, and M, which stand respectively for 1, 5, 10, 50, 100, 500, and 1000 in the decimal system. For example II represents 2 and XX means 20. When the value of the letter placed before another letter is lesser, it is subtracted from that letter. Examples, IX is 9 (10-1), and CM is 900 (1000-100). Pretty straightforward once you get the hang of it!