Normal modes were originally defined as the linear, free vibrations of a system with a finite number of degrees of freedom, such as a finite number of mass particles connected by massless springs. Each mode is a simple harmonic vibration at a certain frequency called an "eigenfrequency" or natural frequency. There are as many independent modes as the number of degrees of freedom. An arbitrary motion of the system can be expressed as a superposition of normal modes. Free vibrations of a finite continuum body, such as the Earth, are also called normal modes. In this case, there are an infinite number of normal modes, and an arbitrary motion of the body can be expressed by their superposition. The concept of normal modes has been extended to wave-guides in which free waves with a certain phase velocity can exist without external force. Examples are Rayleigh waves in a half-space and Love waves in a layered half-space. In these cases, however, one cannot express an arbitrary motion by superposing normal modes. See Udias (2002, p. 90-92), Lognonne and Clevede (2002), and Aki and Richards (2002 or 2009, Chapters 7 and 8)