Bi-quinary coded decimal

Bi-quinary coded decimal is a numeral encoding scheme used in many abacuses and in some early computers, notably the Colossus.

[2] The term bi-quinary indicates that the code comprises both a two-state (bi) and a five-state (quinary) component.

Roman numerals use a symbolic, rather than positional, bi-quinary base, even though Latin is completely decimal.

One advantage of one bi-quinary encoding scheme on digital computers is that it must have two bits set (one in the binary field and one in the quinary field), providing a built-in checksum to verify if the number is valid or not.

The bi-quinary encoding of the internal workings of the machine are evident in the arrangement of its lights – the bi bits form the top of a T for each digit, and the quinary bits form the vertical stem.