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).