Except for light, waves are undulations in a material medium. Transverse wave motion occurs when points in the medium oscillate at right angles to the direction of the wave's travel. If a wave travels from left to right, the particles of the medium will vibrate up and down in place. Electromagnetic waves are transverse - electric and magnetic fields that oscillate perpendicular to the direction of motion. Longitudinal waves, also called pressure waves and compressional waves, oscillate parallel to the direction of the motion of the wave. If a wave travels from left to right, the displacement of the medium is also left to right but oscillates in place by compressing and stretching. When a sound travels through a medium, such as air, it causes compressions and rarefactions in the air as it travels. This application is an essential basic tool for students to study and compare transverse and longitudinal waves.