Octadecimal

Octadecimal or base 18 is a positional numeral system using 18 as the radix. Digits in this base can be represented using the Arabic numerals 0-9 and the Latin letters A-H.

Octadecimal is a rarely used base, though the ancient Mayan calendar is sometimes interpreted as a combination of vigesimal and octadecimal numerical systems - developed in order to match the number of days in a year. There are also some specialised computer systems where octadecimal notation has been used for high levels of data compression. As a base, it has poor radix economy, and because its radix falls between twin primes, there are few recurring fractions with short periods.

In octadecimal, all prime numbers apart from 2 and 3 will end in 1, 5, 7, B, D or H.