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.
Since the Roman system assigns numeric values to letters, some people attempt to use it to derive numeric values for words or names by adding up the letters in them which are also numerals. When doing this, it is usual to count J the same as I, and U the same as V, since those letters were equivalent in Roman times. (W should count as two Vs, equal to X in numbering, since that is the origin of the letter.) If one could get a name to equal 666 this way, this could be seen to have religious significance due to the Biblical reference to the "number of the beast".
This exercise can be done with a number of other numbering systems as well; the Greek and Hebrew alphabets had standard numeric assignments used in those cultures, and in modern times there are even more letter-to-number assigments available, including ASCII and Unicode code points and telephone-dial numbers and letters.
- See also Cyrillic alphabet
In the days of typewriters it was impossible to easily switch to a Latin alphabet, so people used substitutions that looked "close enough" to be understood and interpreted as Roman numerals.
These substitutions were needed, because in Russia (and USSR) Roman numerals are commonly used to write a month in dates (thus non-ambiguously separating it from day and year).
This should not be confused with Cyrillic numerals.