Interference can be constructive or destructive.
Interference of two waves at a given location results in the formation of a new wave pattern which has a greater amplitude than either of the two interfering waves. The meeting of a trough of one wave with a trough of another wave results in destructive interference. The only way for two waves to interfere constructively is for a crest to meet a crest or a trough to meet a trough. Experimental evidence supporting the theory has not yet been observed. Crest meeting crest and trough meeting trough are examples of constructive interference.