Roman numerals
Instead of writing the same letter four times, a rule for subtraction is used. The letter is written once, then the next largest Roman numeral is written. When a lower number (such as ) appears before a higher one (such as ), the lower number is subtracted from the higher one. For example, 4 is not written as , but instead as , because  is  (5) minus  (1). The same is done for 9 - it is not written as , but instead as , because  is  (10) minus  (1).