When we designate the maximum number of speakers/channels for each format, such as 5.1 or 6.1, we're referring to the number of front, center, rear, and possibly center-rear speakers (the .1 refers to the subwoofer) that the format can use. Stereo, a.k.a. 2.0, is a two-channel format. Check out the diagram to see how they are set up in a room. Note that even though 7.1 systems use two back-surround speakers, the same channel of information goes to both of them.

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Subjects in Music