When two waves plane-polarized in two perpendicular planes meet out of phase then the wave resulting from the superposition of the two waves will no longer be plane polarized. The following animations present the superposition of two waves that have the same amplitude and wavelength and are polarized in two perpendicular planes but there is a phase difference of 90° and -90° between them. A phase difference of 90° means that when one wave is at its peak then the other one is just crossing the zero line. A phase difference of -90° requires shifting of one wave relative to the other one along their axis so that there is a 3/4 wavelength difference between them.