In the days of mono you only needed one speaker, when stereo arrived you needed two, preferably identical to each other. For 5.1 you need five, again preferably the same, although in practice smaller models can be used for the surrounds as long as they have the same phase characteristics as the Left/Centre/Right (L/C/R). You may also need a sub woofer, or ideally two, to reproduce the .1 signal.

Two cabinets will result in a more evenly spread bass and perfect stereo compatibility. With some large installations it is possible to reproduce the .1 through the L/C/R monitors. There are several popular surround/multi-channel formats, with and without a sub/Low Frequency Effects (L/F/E).1 channel and there are also specialised systems, the largest Quested installation being a 9.4.