the https://en.wikipedia.org/wiki/Method_of_complements article explains a more fundamental piece of information about this operation - i often see articles explaining two's complement, but doesn't say anything about this general 'complement' method (which works for all bases, not just binary).
> In the decimal numbering system, the radix complement is called the ten's complement and the diminished radix complement the nines' complement. In binary, the radix complement is called the two's complement and the diminished radix complement the ones' complement. The naming of complements in other bases is similar. Some people, notably Donald Knuth, recommend using the placement of the apostrophe to distinguish between the radix complement and the diminished radix complement. In this usage, the four's complement refers to the radix complement of a number in base four while fours' complement is the diminished radix complement of a number in base 5.
That makes sense. The names one's complement and two's complement are kind of confusing otherwise, since they actually refer to totally different things, and it's not clear how they would generalize to higher bases.
Thanks for the link, the article mentions usage of complements in machines going back to mechanical calculators and the first example uses 9's complement. Neat!
