The Roman numeral XV is the equivalent of the number 15. In the Roman numeral system, letters represent numerals. The basic letters and the numerals they represent are I equals one, V equals five, X equals 10, L equals 50, C equals 100, D equals 500, and M equals 1,000.

The rules applying to combination of letters prohibit the use of a particular letter more than three successive times within a number. When the value of a letter is the same or larger than the one immediately following it, the numbers are added. In the Roman numeral XV, X is larger than V, so the two are added together for a total of 15. When a letter has a smaller value than that of the letter immediately following it, the smaller value is subtracted from the larger value. In the Roman numeral IV, for instance, the I has a lower value than the V, so the one is subtracted from the five for a total value of four.