When oscillatory motion occurs in a continuous medium, oscillations couple to adjacent parts of the medium, resulting in a wave that propagates through the medium. Waves are described by the size of the motion (amplitude), the rate at which the motion repeats (frequency), and the distance between wave crests (wavelength). Unlike particles of matter, 2 waves can coexist at the same point. When they do, they interfere, their wave amplitudes adding at that point. Wave interference results in a host of useful and surprising phenomena.