Stereo sound means that two separate sounds, the left (L) and right (R), are produced from a pair of speakers. Because stereo broadcasts must also be usable by receivers that cannot reproduce stereo the signal that all receivers can detect consists of the sum of the left and right channels (R L). The difference of the two channels (R - L) is broadcast 38 kHz above the R L signals. Mono receivers can’t detect these very high frequencies. The stereo receiver, though, can and from the R L and R - L signals creates separate R and L signals to be amplified and sent to the corresponding speaker.

FM stations have enough bandwidth to accommodate a third signal. This signal can be sold to users to provide background music for stores or elevators. Education stations can deliver lessons to schools, or broadcasts in a second language.