A crest is the point on a wave with the greatest positive value or upward displacement in a cycle. A trough is the opposite of a crest.
When the crest and the trough of two waves of equal magnitude and frequency intersect or collide when in phase with each other the result is called constructive interference and the magnitudes double (above and below the line), when 180° out of phase the result is destructive interference with each other the resulting wave is the undisturbed line that is in the middle of the diagram having zero amplitude.
See also
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