Nonlinear amplifiers generate harmonics of the input signal along with an amplified copy of the input. These harmonics can be separated into even order - 2nd, 4th, 6th, etc, and odd order - 3rd, 5th, 7th, and so on. Due to the way harmonics are generated in amplifying devices, the even ordered harmonics do not retain the phase of the input signal. Therefore in a push pull or differential amplifier the two amplifying halves will both generate a spectrum of harmonics but the even order harmonics from each half will cancel each other since their phases are the same and the amplifier output is the difference of the halves. The input signal and the odd order harmonics will not cancel since their phases are opposite between the two circuit halves. Push pull/differential amplifiers typically have lower distortion than single ended amplifiers, due to this cancellation of even order harmonics.