Computers use different numeral bases. A digital computer only deals with 0s and 1s, so binary (base 2) is used at the simplest level. Computers group binary digits together and there Hexadecimal (base 16) is used. When systems such as the PDP-8, ICL 1900 and IBM mainframes were employed, Octal numbers (base 8) became widely used. Three binary digits are grouped together to create an Octal number. Similarly, there are usages for various other number bases.