Roman numerals
Roman numerals were used from ancient Roman times through medieval times (until Hindu-Arabic numerals were introduced to the West), and continue to be seen in various places such as copyright notices and Super Bowl numbers where they're considered to have more "gravitas" than ordinary numbers. Mathematics is very difficult with Roman numerals, which is one reason it stagnated in the West compared to the relatively more advanced Arab countries in the middle ages.
Roman numerals use combinations of a few letters which are given numeric values:
I = 1 V = 5 X = 10 L = 50 C = 100 D = 500 M = 1000
Combinations of these letters normally stand for the sum of them (e.g., VII for 7), but there is also a "subtractive" notation which allows a letter to immediately precede a higher letter in the system to represent the higher letter minus the lower one (e.g., IX for 9, which is 10-1).
Sometimes higher numbers are formed with an overbar indicating 1000 times the letter value, or an overbar combined with surrounding vertical bars representing one hundred thousand times it. However, sometimes overbars and underbars simply represent that something is a numeral rather than other uses of letters.
