One sound—whether the bang of a drum or a note struck on a piano or a bird’s chirp—doesn’t become music until a second sound occurs; either at the same time, called harmony; or at another moment in time, called melody; the ordered spacing of those sounds in time called rhythm. Thus all music begins with the second event. The indivisible number of rhythm is two, for it is the space between the first and second beat that sets our musical expectations and tells us when to expect the third, and so on.
