Tones are often attached to a repeater's uplink frequency to control access or limit noise/interference. In the commercial radio applications and invented by Motorola, the term is known as PL, which was short for Motorola Private Line. In the amateur radio community, the non-propriety term of Continuous Tone-Coded Squelch System, or CTCSS, if often used.

These tones were originally analog only (see chart below). Later, a digital method was also introduced known as Digital Coded Squelch, or DCS. These methods are only used on analog systems. Other methods for controlling access are available for different transmission modes and are discussed on their respective pages.

The tone system can also be applied to the uplink tone to limit interference and noise on the repeater's input/uplink frequency. There are many sources of noise that can break the set squelch threshold, even though no one is intending to activate the repeater. Although the radio will still internally “hear” the noise, it will not open the squelch and retransmit the audio until the proper tone had been received. This method can also be used to allow two repeaters on similar frequencies from interfering with one another, typically in an application where a user is intending to activate one repeater and not the other. Both repeaters can have different access tones so that they are not both activated at the same time.

To reduce confusion as to which tone is being referenced in terms of a paired frequency set (duplex with separate input and output frequencies, as repeaters typically use, the downlink tone refers to the tone that the repeater is transmitting on the downlink frequency. If a user prgrams this feature into their radio, it is often known as “Tone Squelch.” It is not required that a user radio program this tone to receive the repeater's downlink frequency.

Confusion sometimes occurs when it is not known from which radio, the user's or the repeater's, that is being requested. Usually, it seems to get turned around when a repeater owner enters that data. Many repeater coordination councils request the repeater's tones in terms of the repeater's transmit and receive tones, which are opposite from the user radio's on a duplex system.

RepeaterBook enters and displays data from the perspective of the remote (user) radio.