You may have seen some SSB-type CB radios advertised as having 120 channels. This is marketing hype. CB radios in the United States can transmit AM (amplitude modulation) signals, or SSB (single sideband, with suppressed carrier) signals. An AM signal consists of two redundant sideband signals that each contain the operator's voice, along with a so-called carrier signal between them. An SSB signal uses only one of the sidebands. The sideband "channels" are actually the upper and lower halves of the 40 regular AM channels. AM signals and SSB signals on the same channel will interfere with each other.
So how does this all work? When you mix together two signals with different frequencies, you produce two new signals — one with a frequency that's the sum of the first two frequencies, the other with a frequency that's the difference between the first two frequencies. For example, mix an audio tone that has a frequency of 1 kHz (one kilohertz, or one thousand cycles per second) with radio carrier signal of 1000 kHz, and you get two new signals, at 999 kHz and at 1001 kHz. The 999 kHz signal is in the lower sideband (LSB), and the 1001 kHz signal is in the upper sideband (USB). The carrier signal — a steady, continous signal of unvarying frequency — sits between the two sidebands, not really carrying anything at all. All it really does is serve as a zero-reference for the sidebands; in the example above, each of the sideband signals is 1 kHz away from the carrier signal. An AM radio receiver mixes the carrier with the sidebands, and out pops the original audio signal. But a substitute reference signal can be created by a circuit in the receiver, so you don't really need to receive the carrier signal to extract the audio. An SSB radio transmitter suppresses the carrier and one of the redundant sidebands, and thus transmits the operator's voice in less than half the bandwidth of an AM signal, and with much more efficient use of energy.