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This description is taken from the attached book page:

Changing the phase difference between two linearly polarised waves gives a variety of resultant polarisations. When the phase difference is 0 or pi, the result is another

linearly polarised wave. Phase differences of pi/2 and 3pi/2 both result in circular polarisations, but with opposite handedness. Phase differences in between these special cases results in the generalised elliptical polarisation.