Surround sound is a technique for enriching the sound reproduction quality of an audio source with additional audio channels from speakers that surround the listener (surround channels). Its first application was in movie theaters. Prior to surround sound, standard theater sound systems had three "screen channels" of sound, emitted by loudspeakers located only in front of the audience: at the left, center, and right. Surround sound adds one or more channels from loudspeakers behind the listener, thus is able to create the sensation of sound coming from any horizontal direction 360° about the listener. There are various surround sound–based formats and techniques, varying in reproduction and recording methods along with the number and positioning of additional channels. The most common surround sound specification, the ITU's 5.1 standard, calls for 6 speakers: Center (C) in front of the listener, Left (L) and Right (R) at angles of 60° on either side of the center, and Left Surround (LS) and Right Surround (RS) at angles of 100–120°, plus a subwoofer whose position is not critical. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and presents a fixed or forward perspective of the sound field to the listener at this location.