the first number before the period is the number of speakers, the second number after the period is the number of subwoofers.
the most common are 2.0, 2.1, 5.1 and 7.1. less common but used are 3.0, 3.1, 4.0 (quadraphonic), 6.1 and 9.1 along with double subwoofer styles such as 5.2, 7.2, 9.2 etcetera.
anything 2.0 or 2.1 is stereo sound (left and right only).
anything 5.1 (technically 4.0 but that is hardly used) or greater is called surround sound and if the speakers are placed right will give you a sense of directional sound
since your other thread was in reference to computers and since you posted in the computer peripherals forum i'm assuming you are talking about computer speakers here. the 3 most common choices are 2.0, 2.1 and 5.1 sets. generally i would not bother with any 7.1 sets since there is not as much content which can make good use of them.
2.0 - good if you do not need much bass. available powered (by ac plug) or unpowered (powered via computer 3.5mm jack). generally the unpowered speakers will have less volume output and are normally cheaper. the unpowered are good for travel such as for laptop while the powered are better overall.
2.1 - adds a subwoofer to the set which gives much needed low end output. this is the most common setup and is good for people who want decent sound but do not have much room for a full speaker set.
5.1 - if you want surround sound , this is what you want provided you have the room to properly set up and space the speakers around you.
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