and 7.1 refer to the number of audio channels present in a surround sound home theater setup. A 5.1-channel home audio setup includes front and rear speakers plus one or more center speakers. 7.1 adds two more speakers or output channels, usually floor or ceiling rear speakers. Systems such as Dolby Pro Logic II and Dolby Atmos split audio signals into channels corresponding to the speakers in a true surround sound home theater setup.