Method of complements
（重定向自Radix complement）
![Complement numbers on an adding machine c. 1910. The smaller numbers, for use when subtracting, are the nines' complement of the larger numbers, which are used when adding.](/Images/godic/202502/05/Complement_numbering_gnangarra3024.JPG")
![Comptometer from the 1920s, with nines' complements marked on each key](/Images/godic/202502/05/Comptometer1920Model3024.jpg")
In mathematics and computing, the method of complements is a technique used to subtract one number from another using only addition of positive numbers. This method was commonly used in mechanical calculators and is still used in modern computers.
The nines' complement of a number is formed by replacing each digit with nine minus that digit. To subtract a decimal number y (the subtrahend) from another number x (the minuend) two methods may be used: