Damaged audio cassette, with twisted or 'chewed' tape. This could be caused by a malfunction in the cassette or the cassette player. The tape can often be wound back into position by manipulating the reels, however there may be deterioration to the sound quality. The plastic casing protects an oxide-coated magnetic tape wound around two reels. Either side of the tape can contain audio data either as one monophonic track or a pair of stereo tracks. Data is retrieved by playing one side of the cassette then turning it over to play the other side. The cassette was a popular format for purchasing and listening to music from the late Seventies to early Nineties but was then largely replaced by the technically superior and more resilient compact disc (CD).