A wave is a self-sustaining shivering of a medium of transmission, displacing between two points transporting energy and momentum. This movement is called oscillatory movement and advances through the vibration - not displacement - of the particles of the medium. The direction of propagation of the energy of the wave is called direction or line of propagation. When the particles of the vibrating medium vibrate perpendicularly to the line of propagation the wave is called transverse. Examples of transverse waves are string waves and electromagnetic waves. When the particles of the vibrating medium vibrate in the same direction of the line of propagation, the wave is called longitudinal or compressional. The most common longitudinal waves are sound waves.