Roman numerals are symbols that stand for numbers. They are written using seven basic symbols: I (1), V (5), X (10), L (50), C (100), D (500), and M (1,000). Sometimes a bar is placed over a numeral to multiply it by 1,000. A smaller numeral appearing before a larger numeral indicates that the smaller numeral is subtracted from the larger one. This notation is generally used for 4s and 9s; for example, 4 is written IV, 9 is IX, 40 is XL, and 90 is XC.
Roman numerals were developed around 500 b.c.e. at least partially from primitive Greek alphabet symbols that were not incorporated into Latin. Using mainly addition, they are read from left to right. Historians believe the long usage of Roman numerals is due to a number of factors, including the widespread influence of the Roman Empire, tradition, and the fact that the system had many advantages over other European systems of the time. For example, the majority of users had to memorize only a few symbols and their values. You can see Roman numerals every where today in the appendices in books, in the credits of feature films, on building faces for dates, and on watches and clocks.