XIV is X + IV, which means 10 + 4.
We are only allowed to write three of the same numeral at once.
Here are the Roman numerals 11 to 20 formed by writing an X numeral in front of the numeral that is 10 less in value.
Mixed Roman and Arabic numerals are sometimes used in numeric representations of dates especially in formal letters and official documents, but also on tombstones.
Some modern sources describe Vinculum as if it were a part of the current "standard".
Alphabetic symbols for larger numbers, such as Q for 500,000, have also been used to various degrees of standardization.