octal and HEXADECIMAL number systems
The octal number system is a base eight number system. As such, it carries over every eighth number rather than the tenth that we're accustomed to. It is represented as a series of the numbers zero through seven with each place value in creasing by a power of eight (8^0 - 8^1 - 8^2...).
The hexadecimal number system is a base sixteen number system. As such, it carries over every sixteenth number rather than the tenth that we're accustomed to. It is represented as a series of the numbers zero through 15 with A representing 10, B 11, C 12, D 13, E14, and F 15. Each place value in creases by a power of sixteen (16^0 - 16^1 - 16^2...).