Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that creates an illusion of multi-directional audible perspective. This is usually achieved by using two or more independent audio channels through a configuration of two or more loudspeakers (or stereo headphones) in such a way as to create the impression of sound heard from various directions, as in natural hearing. Thus the term 'stereophonic' applies to so-called 'quadraphonic' and 'surround-sound' systems as well as the more common two-channel, two-speaker systems. It is often contrasted with monophonic, or 'mono' sound, where audio is heard as coming from one position, often ahead in the sound field (analogous to a visual field). Stereo sound has been in common use since the 1970s in entertainment systems such as broadcast radio, TV, recorded music, internet, computer audio, and cinema. The word stereophonic derives from the Greek στερεός (stereós, 'firm, solid') + φωνή (phōnḗ, 'sound, tone, voice') and it was coined in 1927 by Western Electric, by analogy with the word 'stereoscopic'. Stereo sound systems can be divided into two forms: the first is 'true' or 'natural' stereo in which a live sound is captured, with any natural reverberation or ambience present, by an array of microphones. The signal is then reproduced over multiple loudspeakers to recreate, as closely as possible, the live sound.