This dynamic distinction it is that gives rise to the two fundamental classes of rhythm, the duple and triple.

In duple rhythm the accent is followed by one unaccented or lighter beat, so that regular alternation of heavy and light pulses prevails incessantly.

In triple rhythm the accent is followed by _two_ lighter beats, creating similarly constant, but _irregular_ alternation of heavy and light pulses.
[Illustration: Duple and Triple Rhythm] This distinction is so significant and so striking, that the music lover who is eager to gain the first clues to the structural purpose of a composition, should endeavor to recognize which one of these two rhythmic species underlies the movement to which he is listening.

It is fairly certain to be one or the other continuously.

Of duple measure, the march and polka are familiar examples; of triple measure, the waltz and mazurka.