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The principle of superposition may be applied to waves whenever two (or more) waves travelling through the same medium at the same time. The waves pass through each other without being disturbed. The net displacement of the medium at any point in space or time, is simply the sum of the individual wave displacements. A travelling wave whose amplitude depends on the phase (ϕ). When the two waves are in-phase, they interfere constructively and the result has twice the amplitude of the individual waves. When the two waves have opposite-phase, they interfere destructively and cancel each other out. The waves in this picture are mostly flat so it must be destructive.