Stereo speaker systems consist of two speakers, one for each sound channel. A 2.1 system denotes two satellites and one woofer unit. Satellites can be added to the system to create a 4.1 system, consisting of two satellites each in the front and back. A 5.1 system adds a centre speaker in the front to the 4.1 system. A 6.1 system adds a central back satellite to the 5.1 system. A 7.1 system adds two speakers at the same level as the listener to the 5.1 setup.