Citizens band radio (also known as CB radio) is, in many countries, a system of short-distance radio communications between individuals on a selection of 40 channels within the 27 MHz (11 m) band. Citizens band is distinct from the FRS, GMRS, MURS, and Amateur Radio Service ("ham" radio) systems. In many countries, CB operation does not require a license, and (unlike amateur radio) it may be used for business or personal communications. Like many other two-way radio services, citizens band channels are shared by many users. Only one station may transmit at a time; 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" during a lull in the conversation. This informs people using the channel that others are waiting.