From Just Solve the File Format Problem
Ones' complement, two's a crowd. Ones' compliment is a binary system in which the negative of a number is formed by inverting all its bits, in distinction from Two's complement, where one is added to this inversion to produce the final result. In ones' complement notation, there is both a positive and a negative zero; two's complement avoids this and makes it possible to express one more negative number than positive (which is why 16-bit signed integers can range from -32768 to 32767 in two's complement but only -32767 to 32767 in ones' complement).
Some old computer models used ones' complement, but most modern systems use two's complement.