Bi-quinary coded decimal is a numeral encoding scheme used in many abacuses and in some early computers, including the Colossus. The term bi-quinary indicates that the code comprises both a two-state (bi) and a five-state (quinary) component.

Several different representations of bi-quinary coded decimal have been used by different machines. The two-state component is encoded as one or two bits, and the five-state component is encoded using three or five bits. Some examples are:

• IBM 650 – 7 bits (two ‘bi’ bits: 0 5 and five ‘quinary’ bits: 0 1 2 3 4) with error checking (exactly one ‘bi’ bit and one ‘quinary’ bit set in a valid digit); in the picture of the front panel below, the bi-quinary encoding of the internal workings of the machine are evident in the arrangement of the lights (active bits are just visible)

Value	05-01234 Bits	IBM 650 front panel
0	10-10000
1	10-01000
2	10-00100
3	10-00010
4	10-00001
5	01-10000
6	01-01000
7	01-00100
8	01-00010
9	01-00001

• UNIVAC Solid State – 4 bits (one ‘bi’ bit: 5 and three binary coded ‘quinary’ bits: 4 2 1) with 1 parity check bit

• Univac LARC – 4 bits (one ‘bi’ bit: 5 and three ring counter coded ‘quinary’ bits) with 1 parity check bit

See also

• Binary
• Binary-coded decimal