Roman numerals are written from 7 symbols combined with each other. The symbols were originally graphic signs now confused with the letters I, V, X, L, C, D, M. Each symbol represents a numeric value, 1, 5, 10, 50, 100, 500, 1 000, respectively.

Other numbers are formed by combining several symbols together, respecting the rules of addition or subtraction. In Roman numeration, numbers are written from left to right. As a general rule, the symbols are arranged by decreasing value.

The value of the number is deduced by adding the values associated with each symbol: MMDCCLXXVII represents the number 2 777 (1 000 + 1 000 + 500 + 100 + 100 + 50 + 10 + 10 + 5 + 1 + 1).

Subtraction rules only apply to symbols I, X and C. These can be placed before 2 symbols of higher value and only 2. In this case the value of I, X or C is subtracted from that of the symbol it precedes.

- The symbol I can be placed in front of the symbols V and X: IV represents the number 4 (5-1), IX represents the number 9 (10-1).
- The symbol X can be placed in front of the symbols L and C: XL represents the number 40 (50-10), XC represents the number 90 (100-10).
- The symbol C can be placed in front of the symbols D and M: CD represents the number 400 (500-100), CM represents the number 900 (1 000-100).