| stereo - from the Greek sterios, meaning solid. Nothing about 2.
Every sound we (even those among us with hearing in only one ear) hear is from a 3-dimensional space (unless we are in an anechoic chamber). An acquisition/delivery format must be adequate for a 3-dimensional space.
2-channel stereo can do this with some material if it is recorded and played correctly. Playback is critical, and the phantom image can only be portrayed if the listener is positioned dead-center between the two speakers. Even then, the phantom image has time and frequency issues that a single speaker will not have (look up Head Related Transfer Function as a start).
Making stereo from LCR instead of LR gives a more-accurate representation of any front soundfield. And there are now three phantom images instead of one (L-C and C-R being less solid than LR, but they are there).
5-channel stereo gives us ten phantom images! The L-Ls, C-Rs, etc., are perceived differently by our ear/brain mechanism than the L-R, but they are there. 5-ch also adds the ability to have smooth dynamic pans from left to right, and diffuse or discrete elements from the rear. These are all great tools, and fewer channels can be used when appropriate (C-only is the ultimate way to present mono recordings, for example).
5.1 (the .1 LFE channel has no bearing on stereophonic sound, and probably should never be used in a music-only mix anyway) was developed as the minimum configuration to deliver a convincing 3-dimensional image. Our ears do not perceive the phantom image from the right or left (made by L-Ls and R-Rs), so 7.1 systems with discrete speakers to fill this gap can help; our ears do not perceive the Cs phantom made by Ls-Rs well, so 6.1 systems with a discrete Cs can be used. No current widely-used system, from 1.0 to 7.1, can deliver much of a height-image, so height channel(s) can be used. 10.2 as developed by Tomlinson Holman have all of these, so this might be the logical next step. Where we stop between 2.0 and ~.x is anyone's guess. |