The MSB has a negative weight of −2ⁿ⁻¹. Negation: Invert the bits, then add 1.

+13 = 0000 1101
Invert: 1111 0010
+ 1: 1111 0011 = −13

Advantages: Only one zero · Subtraction = addition of the negative · no special case in hardware.

8-bit range: −128 to +127. Asymmetric: −(−128) cannot be represented!