In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process. While all forms of modulation have sidebands by definition, it is most commonly discussed in amplitude modulation (AM).

Amplitude modulation of a carrier wave normally results in two sidebands. The frequencies above the carrier frequency constitute the upper sideband (USB), those below the carrier frequency, constitute the lower sideband (LSB). In conventional AM transmission, the carrier and both sidebands are present, sometimes called double sideband.

If part of one sideband and all of the other remain, it is called vestigial sideband, used mostly with television broadcasting, which would otherwise take up an unacceptable amount of bandwidth. Transmission in which only one sideband is transmitted is called single-sideband transmission (SSB, particularly USB and sometimes LSB), used mainly in amateur radio.

In many cases, paticularly in SSB amateur radio, a suppressed carrier is used, significantly reducing the amount of electrical power used (by up to 12 times), while still leaving the audio or other information present in the sideband. In this case, a beat frequency oscillator must be used at the receiver to reconstitute the signal.

Sidebands are also what cause interference to adjacent channels, and therefore they must be suppressed by filters, either before or after modulation (or often both). In frequency modulation (FM), subcarriers above 75 kHz are suppressed to a small percentage of modulation, and are prohibited above 99 kHz altogether, in order to protect the ±75 kHz normal deviation and ±100 kHz channel boundaries.

Source: partly from Federal Standard 1037C in support of MIL-STD-188