The BCD code uses a 4-digit binary number to represent a decimal number. Usually 8421 yards (there are other such as 2421 yards and the remaining 3 yards).

Here is an explanation with the 8421 code.

Four digits in binary. Corresponding to the numbers 8, 4, 2, 1 respectively.

For example: 1010 then the corresponding number is 8 0 2 1 plus one is eleven

For example: 0101 then the corresponding number is 0 4 0 1 plus five is together

The same reason

2421 code is the corresponding number 2, 4, 2, 1

The remaining 3 yards is using 8421, but after using it, it is reduced by 3.

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