Rhythm (from the Greek rhythmós, “measured flow,” “symmetry”), the regular metrical movement or recurrence of a phenomenon, is a key element in the subdivision of time. It divides up astronomical, “natural” (e.g. seasons), physical (e.g. the pendulum), biological (e.g. cell division, heartbeat, breath), psychological (mental processes congruent to biorhythms), and cultural processes (see Periodicity). It also forms a fundamental structural element of complex cultural phenomena, such as lang…