The convention by which bit patterns with high bit 0 represent positive numbers from 0 to 2^{n-1} - 1 directly, while bit patterns with high bit 1 represent negative numbers from 0 to -(2^{n-1} - 1), n being the word size of the machine, and the numeric complement of a number is its ones' complement
Computers that use ones' complement arithmetic have two representations for zero, all 0's and all 1's.