Citizens band radio
Like many other two-way radio services, citizens band channels are shared by many users. Only one station may transmit at a time. All other stations must listen and wait for the shared channel to be available. It is customary for stations waiting to use a shared channel to broadcast the single word "break" (or "breaker") during a lull in the conversation. This lets other people using the channel know that others are waiting. In the US, channel 19 is most often used by truckers. Channel 9 is reserved for emergencies.