Phase inversion

Note: Phase inversion may occur with a random or periodic, symmetrical or non-symmetrical waveform, although it is usually produced by the inversion of a symmetrical periodic signal, resulting in a change in sign. A symmetrical periodic signal represented by f (t ) = A e^jωt , after phase inversion, becomes f ₁(t ) = Ae^{j(ωt +π)}, where t is time, A is the magnitude of the vector, ω is angular frequency (ω = 2πf ), where f is the frequency and π ≈3.1416 and e ≈ 2.7183. The algebraic sum of f (t ) and f ₁(t ) will always be zero.

Source: from Federal Standard 1037C