CMXCIX is a Roman numeral that represents the number 999. In the Roman numeral system, letters are used to signify values: C stands for 100, M for 1000, X for 10, and I for 1. The combination of these letters in CMXCIX indicates that you take 1000 (M) and subtract 1 (I) from 1000, then subtract 10 (X) from 100, resulting in 999. Roman numerals, including CMXCIX, were widely used in ancient Rome and are still seen today in various contexts, such as on clock faces, in book chapters, and for naming events like the Super Bowl. Understanding Roman numerals can enhance appreciation for historical texts and artifacts.