The Aiken code is a BCD code in which the pseudotetrade is in the middle of the value range. The place value system of the four bits of the Aiken code represents the values 2-4-2-1, which is why this code is also called 2-4-2-1 code.
The first decimal digits up to the decimal number 4 correspond Aiken-coded to those of the BCD code, the following 6 values form the pseudotetrade and the following five values form the digits 5 to 9. Each Aiken-coded number can be transferred to the BCD code.
The Aiken code is self-complementing, which means that swapping the 0 and the 1 gives the complement. The Aiken code thus offers advantages in the subtraction of dual numbers, which can be done by simply forming the complement of 9 and then adding.
The Aiken code is named after Howard Hathaway Aiken, a professor of applied mathematics at Harvard University in Cambridge.