Generally, Roman numerals are written in descending order from left to right, and are added sequentially, for example MMVI (2006) is interpreted as 1000 + 1000 + 5 + 1.
Certain combinations employ a subtractive principle, which specifies that where a symbol of smaller value precedes a symbol of larger value, the smaller value is subtracted from the larger value, and the result is added to the total. For example, in MCMXLIV (1944), the symbols C, X and I each precede a symbol of higher value, and the result is interpreted as 1000 plus (1000 minus 100) plus (50 minus 10) plus (5 minus 1).
A numeral for 10n (I, X, or C) may not precede a numeral larger than 10n+1, where n is an integer. That is, I may precede V and X, but not L or C; X may precede L or C, but not D or M. The numerals 5×10n (V, L, or D) may not be followed by a numeral of greater or equal value. Any symbol that appears more than once consecutively may not be followed by a symbol of larger value.