The Romans used seven letters of the alphabet to depict numbers. Each letter was assigned a fixed value as follows:

Arranging these letters in different configurations gave rise to different values of numbers. Generally, numbers placed side to side were added but if I, X or C was to the left of a larger numeral then the former would be subtracted from the latter. Another rule stated that subtraction could be done only if the smaller number is no less than one-tenth the magnitude of the larger number. Hence 'nine' would be written as 'IX' but 'ninety-nine' could not be written as 'IC' (it should be written as 'XCIX'). This number system was unique in that it did not use the place value system developed in India and China, yet it did away with using a specific character for every number above nine. 

The Roman numeral system is still used for non-arithmetic purposes. For example, copyright dates for films and television programmes are generally written in this system. It is also used for clock faces; however, in most clock faces the number '4' is written as 'IIII' and not 'IV' as it normally is. Though simple addition and subtraction are simple using this system, complex functions such as multiplication are more difficult. This was the main reason why the Roman numeral system was phased out by the Arabic numeral system.