A Citizens Band Radio (CB) is a mobile communication tool for private use. It can be powered by either a 12-volt or a 110-volt source and has a range of 15-30 miles, depending on the user's location and the quality of radio equipment. Multiple users with CB units tuned to the same radio frequency channel can communicate with each other. A standard CB radio includes a combined transmitter-receiver (i.e., a "transceiver") and an antenna. In the United States, there are 40 radio frequencies designated for CB use, but there is little privacy since anyone can use these channels. In some areas, the traffic on these channels is heavy and another form of communication might have to be used. Channel 9 is designated as a reserve frequency for emergency personnel.