Hexadecimal

Hexadecimal is a base 16 system for simplifying how binary is represented, it's main purpose is to simplify binary values, so for example, 11, which is the letter B is the simple representation of 1011.

The Hexadecimal sequence starts at a double digit number such as 10 which is A and continues B, C, D and so on to 15.

This is used as you cannot use double digit numbers when counting up in hexadecimal, only single digit or letters.