Overlapping waves of identical frequency and amplitude produce stable interference patterns. A, Waves overlap in phase— peaks coincide with peaks and troughs with troughs—producing a resultant wave of twice the amplitude. B, Waves overlap out of phase—the peak of one wave coincides with the trough of the other—and the waves cancel. C, Waves partially overlap—neither completely in nor out of phase—producing a wave of intermediate amplitude (between zero and twice the amplitude).