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For radio operation in the frequency range of 30 mc downward, ordinary amplitude modulation (AM) has been most widely used. In order to overcome certain known deficiencies of AM, variations of AM have been evolved. One of these, single sideband operation with the carrier suppressed at the transmitter (SSB), has proved its superiority over AM in transatlantic operations for nearly twenty-five years. A notice of proposed rule-making by the FCC, in 1955, to make mandatory the use of SSB below 25 mc in certain services, quickly caused two other systems to be put forward for consideration. These were 1) a system of transmitting both sidebands with the carrier suppressed at the transmitter (DSB), and 2) a form of single sideband with the carrier transmitted, called Compatible Single Sideband (CSSB). This paper deals mostly with a comparison of SSB and DSB, with emphasis on the applications to the aeronautical mobile service.