Roman numeral is an ancient Roman numeric system of notation based on the combination of letters of alphabet to symbolize the sum of their values. The Roman numeral system is also known as Latin numeric system, is only for natural number and does not contain zero.
Roman numerals Symbols :
Roman Numeral Symbols

Basic principles for reading Roman numerals:
1. If a numeral is placed after another lower denomination numeral, those numerals are added together.
    Example: $VI = 6 (V + I),  LXX = 70,  MCC = 1200$, etc.
2. If a numeral is placed before another letter of lower denomination, the smaller numerals are subtracted from the larger numerals.
     Example: $IV = 4 (V - I),  XC = 90,  CM = 900$ ,etc.   

3. When a bar is placed over a numeral, it increases the value of numeral by 1,000 times. 
    Example: $ X = 10,  X = 10,000$
                   $XV = 15,   XV = 15,000$
4. A numeral is repeated once or twice, repeats its numeral value many times.
     Example: $CC = 200,   XXX = 30$