The numbers that we normally use (1, 2, 3 etc) are called "Arabic numerals". But we sometimes use another system for writing numbers - "Roman numerals". The Romans used letters of the alphabet to represent numbers, and you will occasionally see this system used for page numbers, clock faces, dates of movies etc.
The letters used in Roman numerals are:
We can use upper-case letters (capitals) or lower-case letters (small letters) when writing Roman numerals. So the following numbers are exactly the same: XVIII = xviii = 18
In general, letters are placed in decreasing order of value, eg XVI = 16. Letters can be repeated one or two times to increase value, eg XX = 20, XXX = 30. Letters cannot be repeated three times, so XXXX is not used for 40. In this case, XL = 40 (50 minus 10).
Look at these examples of Roman numerals in use:
Significant Roman numerals from one to a thousand
Arabic Numeral > Roman Numeral Converter
You can use the tool below to convert an Arabic numeral into a Roman numeral: