They use a few symbols that represent numerical values:
| Roman | I | V | X | L | C | D | M |
| value | 1 | 5 | 10 | 50 | 100 | 500 | 1000 |
If a symbol is less than or equal to the one on its right, add its value to the previous symbol. If it is less, subtract its value.
Examples:
| VI | 1+5=6 (Remember, compute from right to left) |
| IV | 5-1=4 |
| XX | 10+10=20 |
| XIX | (10-1)+10=19 |
| XIII | 1+1+1+10=13 |
| XIV | (5-1)+10=14 |
| MCDXCII | 1+1+(100-10)+(500-100)+1000=1492 |
| MDCCLXXVI | 1+5+10+10+50+100+100+500+1000=1776 |
| MMII | 1+1+1000+1000=2002 |
Imagine trying to do math with these clumsy tools. For practical daily uses, Romans used an abacus (a counting frame with beads strung on wires.) These were the symbols in use in the early middle ages of Europe, until they were replaced by the "Arabic" numerals, an elegant system of numbering that came from India, by way of Islamic Spain.
