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, 1000, 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 2777 (1000 + 1000 + 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.