Aiken code

The Aiken code (also known as 2421 code)^[1][2] is a complementary binary-coded decimal (BCD) code. A group of four bits is assigned to the decimal digits from 0 to 9 according to the following table. The code was developed by Howard Hathaway Aiken and is still used today in digital clocks, pocket calculators and similar devices^{[citation needed]}.

Aiken code
Digits	4[1][2]
Tracks	4[1][2]
Digit values	2 4 2 1[1][2]
Weight(s)	0..4[1][2]
Continuity	no
Cyclic	no[1][2]
Minimum distance	1[1][2]
Maximum distance	4[1][2]
Redundancy	0.7
Lexicography	yes[1][2]
Complement	9[1][2]
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The Aiken code differs from the standard 8421 BCD code in that the Aiken code does not weight the fourth digit as 8 as with the standard BCD code but with 2.

Aiken code (symmetry property)

Aiken code in hexadecimal coding

The following weighting is obtained for the Aiken code: 2-4-2-1.

One might think that double codes are possible for a number, for example 1011 and 0101 could represent 5. However, here one makes sure that the digits 0 to 4 are mirror image complementary to the numbers 5 to 9.

Aiken code
Decimal digit	Aiken 2 4 2 1 code[1][2]
0	0 0 0 0
1	0 0 0 1
2	0 0 1 0
3	0 0 1 1
4	0 1 0 0
5	1 0 1 1
6	1 1 0 0
7	1 1 0 1
8	1 1 1 0
9	1 1 1 1

See also

• Excess-3 code
• Gray code
• O'Brien code type I