The number 666 is written as DCLXVI in Roman numerals. The letters stand for 500, 100, 50, five and one respectively. There are only seven symbols in Roman numerals.
The symbols cannot be repeated more than three times. This is why 40 cannot be written as XXXX but must be written as XL. This means 10 must be subtracted from 50 as the smaller symbol is to the left of the larger symbol.
Similarly, sixty is written as LX. In this case, the smaller symbol is to the right of the larger symbol and so it must be added.
Roman numerals were developed by the Romans and then replaced by the Arabic system of numbers.