"The simple mechanical system of the classical harmonic oscillator underlies important areas of modern physiccal theory. ... The concept of degeneracy arises in the two-dimensional oscillation of a square plate or diaphragm. Three-dimensional harmonic oscillation relates to oscillatory modes in the Rayleigh-Jeans equation ... Vibration of a macroscopic three-dimensional crystal is treated by Debye's theory ... Harmonic oscillator theory is important when it succeeds and also when if fails, as we shall see in the motivation to find a theory of radiation that we now call the quantum theory ..."