To avoid potential interference between broadcast television stations, regulatory agencies do not allow adjacent TV channel operation in a given geographic area, thus guaranteeing the existence of certain "unused" TV channels in that area. For example, in the US, if a locally active TV channel 9 exists, channels 8 and 10 will be vacant. These vacant channels can then be used by wireless microphone systems with little concern for television interference. Many manufacturers pre-select groups of wireless microphone system frequencies based on the availability of vacant TV channels. They also have information on TV channel distribution throughout the US. It is usually sufficient to indicate the destination of the wireless equipment at the time that it is specified in order to avoid broadcast television interference.