Hexadecimal
Unlike modern computers, many early computers had six-bit bytes.   Programmers of those systems typically used an alternate bit grouping scheme called octal.  Each octal digit efficiently represents three bits, and a six-bit byte can be represented as two octal digits.  Three bits, each being on or off, can represent the eight numbers from 0 to 7: 000 = 0; 001 = 1; 010 = 2; 011 = 3; 100 = 4; 101 = 5; 110 = 6 and 111 = 7.