With respect to home playback, the terms 5.1, 6.1, and 7.1 mean that there are five, six, or seven main speakers, plus a subwoofer, in the playback system. (The subwoofer reproduces the LFE channel recorded on 5.1 soundtracks, plus any bass the main speakers cannot handle.) The difference is in the number of surround speakers: two in a 5.1 system, three in a 6.1 system, and four in a 7.1 system.
Obviously, a 5.1-channel soundtrack can be played on a 5.1-speaker system. But it is not always understood is that it can also be played on a 6.1- or a 7.1-speaker system. To do this, the two surround signals on the 5.1 soundtrack are spread across the three or four surround speakers. This distribution can be accomplished by a Dolby Digital EX decoder, a THX Surround EX decoder, or other proprietary methods provided in home theater equipment by various manufacturers.
So the number (i.e., 5.1) describing the soundtrack does not have to match the number applied to the speaker system.
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