The Fourier transform, named for Joseph Fourier, is an integral transform that expresses a mathematical function of time as a function of frequency. For instance, the transform of a musical chord made up of pure notes without overtones, expressed as loudness as a function of time, is a mathematical representation of the amplitudes and phases of the individual notes that make it up. The function of time is often called the time domain representation, and the function of frequency is called the frequency domain representation. The inverse Fourier transform expresses a frequency domain function in the time domain. Each value of the function is usually expressed as a complex number (called complex amplitude) that can be interpreted as an absolute value and a phase component. In the case of a periodic function, such as a musical tone (possibly with overtones), the Fourier transform can be simplified to the calculation of a discrete set of complex amplitudes, called Fourier series coefficients.