Roman numerals 1~20 are: one, two, three, four, five, six, seven, eight, nine, eight, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen, eight.

Roman numerals are used in Europe before the introduction of Arabic numerals (actually Indian numerals), but they are rarely used now. It came into being later than the numbers in China and Oracle Bone Inscriptions, and even later than the Egyptian decimal numbers. However, its appearance marks the progress of an ancient civilization.

Extended data:

Historical origin

Roman numerals are more than 2000 years earlier than Arabic numerals and originated in ancient Rome.

The most commonly used method of expressing 4 in ancient Rome was IIII, so the design style of IV has been strongly protested by "orthodoxy".

James O 'Donnell, professor and provost of classical philology at Georgetown University, said that although the ancient Romans sometimes wrote 4 as IV to save space, this abbreviation did not become popular until the Middle Ages. Actually, it's not formal.

There is also a theory put forward by watch experts that the ancient Romans used IV as the abbreviation of the name of JUpiter, the king of gods (in classical Latin, J and I are I, U and V are V, so IV is the abbreviation of Ju and Jupiter), because they didn't want the name of God to look like a number, which is a "taboo".