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 crests and troughs 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: the resulting wave is the undisturbed line having zero amplitude.
