Only pure sinusoidal oscillations consist of a single frequency, the fundamental, when viewed in terms of frequency, as shown by the Fourier transform
.sinusoidal signals, square-wave signals, pulses or sawtooth signals and all other signals are composed of the fundamental and a greater or lesser number of harmonics. The fundamental of an oscillation and its harmonics are called "harmonics". The fundamental is the 1st harmonic, the 1st harmonic is the 2nd harmonic, the 2nd harmonic is the 3rd harmonic, and so on. Subharmonics are below the fundamental in frequency, their frequencies are 1/2, 1/3, 1/4, etc. of the fundamental.