Con Moto: Slave to the RhythmIf rhythm is the heartbeat of music, it's often an irregular one. Music gets its rhythmic energy not from the regular repetition of a pulse, but by the way the notes and sounds weave themselves around such a pulse. That pulse is not the rhythm but the metre, much the same as what we colloquially call the 'beat'– the regular division of time into groups of instants each separated by equal intervals. The notes themselves don't have to sit at those instants: sometimes notes fall off the beat, sometimes they are sustained over the beat, sometimes they crowd the gaps between beats.
We are natural seekers of metre: we have an in-built instinct to group together regular pulses into small clusters. If we hear a series of identical pulses, we'll typically hear it as being divided into groups. This grouping instinct is found even in young babies.
Western music uses mostly simple metres: recurring groups of two, three or four pulses, or sometimes six. Cultures outside the main Western tradition may use more complex metres: Balkan music often employs groupings of five and seven pulses, for example, generally subdivided into groups of twos and threes. The prevalence of two- and four-beat metres in Western music is sometimes said to stem from the natural rhythms of marching and dancing in bipedal humans, but in fact it is nothing more than a cultural convention: Greeks have no problem dancing to seven beats.
Our ability to pick out pulse and rhythm from a piece of music leans heavily on the cognitive grouping mechanisms called the Gestalt principles, our mental toolkit for finding (and sometimes imposing) pattern on sensory data. So determined is our search for regularity that we may banish ambiguities: we simply don't hear things that don't fit the pattern, or mentally shift them until they do. These instincts are manipulated in some modern experiments in rhythm in Western music, ranging from the fragmentation of pulse by irregular emphasis in Stravinsky's Rite of Spring to the use of simultaneous rhythms that don't overlap precisely by American composer Steve Reich.