The complement of a number is the difference between that number and 10000. . . (to as many zeros as the original number has). In the decimal system, the complement of 354 is 1,000 - 354, or 646. (Binary numbers have corresponding complements as well.) Computers usually subtract by adding the complement of a number, for example, 421 - 354 is computed as 421 + 646, or 1,067. If the machine's registers have only three places, the "1" at the left drops out, giving the true difference 67. A variation of this scheme is to take the difference between each digit and 9, giving the so-called nines complement, instead of the "tens complement" just described. That has the advantage of not requiring any borrowing of digits to form the complement, but a 1 must be added (the so-called end-around carry) to give the correct answer for subtraction.