I = one, V= 5, X =10, L=50, C = 100, D = 500, M=1000. 9 for example is 1 less than 10 therefore it is written IX, 11 is one more than 10= XI.
Roman numerals are a numeral system used in ancient Rome and are still occasionally used today for decorative purposes or in some specific contexts. The basic symbols in the Roman numeral system are I, V, X, L, C, D, and M, representing numbers 1, 5, 10, 50, 100, 500, and 1000, respectively. Roman numerals are typically written as a combination of these symbols, with certain rules for their arrangement and repetitions to form larger numbers.
12 = XII in Roman numerals
18 in roman numerals looks like XVIII
The Roman numerals are XLV.
300 would be represented by 'CCC' in roman numerals.
MDCCCXCV
MMDCCCLXXXVI
MCMLXXXV
MCMLXV
XXI
DCLXVI
MCMLII
MCMLXXXIX