Plainsong is a single, unaccompanied melodic line. The vocalists sing in unison, none diverging from the shared idea. An antiphon is a song in which the two independent choirs interact by singing alternate musical phrases. If one choir asks a question, the other will answer. Eventually the voices blend into harmony. A canon is a composition in which a series of repetitions develop into a single melody. But the individual voices may interpret the melody in contrasting ways: it might be inverted, or in reverse, or in a different pitch. The combination results in a rich and dense swirl of sound.