CBS (an initialism of its former name, the Columbia Broadcasting System) is an American broadcast television network that originated as a radio network in September 1927, and expanded into television in July 1941. Throughout its history, the network has had many owned-and-operated and affiliated stations. The station's advertised channel number follows the call letters. In most cases, this is their virtual channel (PSIP) number, which may match the channel allocation that the station originally broadcast on during its prior affiliation with the network.