CB radio in the United States

In the United States, the CB Band Radio Service, or Citizens' Band ("CB"), known as the Class D Citizens' Radio Service before 1976, is one of several personal radio services defined by the FCC's Part 95 rules. It is intended to be a two-way voice communication service for use in personal and business activities of the general public, and has a reliable communications range of several miles, though the range is highly dependent on type of radio, antenna and propagation. Class A and B are no longer in existence (the frequencies were folded in to the GMRS radio service). The former Class C, now known as the Radio Control Radio Service (RCRS) is a Radio Control ("R/C") model service and is covered further down in this article.
CB radio is most frequently used by long-haul truck drivers for everything from relaying information regarding road conditions, the location of speed traps and other travel information, to basic socializing and friendly chatter. CB radio is also frequently used on larger farms for communication between machinery operators. 
Eligibility.
There are no age, citizenship, or license requirements to operate a CB radio in the United States, and the service falls under the "License by Rule" part of the FCC rules (basically, if one follows the rules one is considered licensed). Operators may use any of the authorized 40 CB channels; however, channel 9 is used only for emergency communications or for traveler assistance and the higher number channels are almost exclusively SSB modulation. Use of all channels is on a shared basis. However, foreign governments and their representatives are not eligible to use citizens' band radio within the United States.
Operation is permitted anywhere within the United States and its territories or possessions; as well as anywhere in the world except within the territorial limits of areas where radio services are regulated by a foreign government, or another U.S. agency such as the Department of Defense.
Transmitters must be FCC certified and may not be modified, including modifications to increase output power or to transmit on unauthorized frequencies. Output power is limited to 4 watts for AM transmitters and 12 watts peak envelope power for single sideband (SSB) transmitters. The antenna may not be more than 20 feet (6.1 m) above the highest point of the structure it is mounted to, or the highest point of the antenna must not be more than 60 feet (18.3 m) above the ground (47 CFR 95.408(c)) if installed in a fixed location.
AM and SSB modulation is used in the USA, some other countries have similar services in their countries that use FM.
Channel assignments.
North American/CEPT frequencies.
To simplify selection of an operating frequency, the Citizens' Band radio spectrum is divided into 40 numbered radio frequency channels from 26.965 to 27.405 MHz, with channels generally spaced 10 kHz apart. Channel numbers are not strictly sequential with frequency; there are gaps for frequencies used by radio-controlled ("R/C") devices.
For historical reasons, there is a gap between channel 22 and channel 23 (which was later filled by channels 24 and 25). 27.235, 27.245 and 27.255 were assigned to Business radio use while the lower spectrum was assigned to the 11-meter amateur (ham) radio band. The 11-meter band became CB Channels 1 to 22, and the last business frequency was channel 23 and shared.
The frequency allocation list shown is supplied by the FCC (Federal Communications Commission) from Part 95 - Subpart D - Rules for Citizens Band (CB) Radio Service.
Channel usage.
Channel 19 is the most commonly-used channel by truck drivers on highways, to the point that some radios even have a dedicated button to bring up channel 19 instantly. In most areas of the U.S. other channels regionally used for this purpose include 10, 17, and 21. Channel 13 is preferred in some areas for marine use and for recreational vehicles.
Several countries reserve a channel for emergency use, for example, channel 9 in the United States. In CB's heyday in the 1970s and 80s, channel 9 was monitored by parties who could relay messages to the authorities, and often monitored directly by the authorities themselves. With the popularity of cellular phones since the 1990s, support for Channel 9 as an emergency channel has diminished, though volunteer organizations such as REACT (Radio Emergency Associated Communications Teams), and private individuals still monitor Channel 9 in some (particularly rural) areas.
Shared radio services.
Remote control.
Among several other services that share the CB frequencies is the former Class C Citizens Band service, renamed to the Radio Control Radio Service (RCRS) in 1976, outlined in Subpart C of the Part 95 rules for radio-controlled ("R/C") devices. No voice transmissions are permitted. It has six channels in the 27 MHz band. Five are unused 10 kHz CB assignments between channels 3-4, 7-8, 11-12, 15-16 and 19-20, and the sixth is shared with Channel 23. R/C transmitters may use up to 4 watts on the first five channels and 25 watts on the last, 27.255 MHz. Some in-house paging systems, and car alarms with a paging feature, also use these frequencies, especially 27.255 where the higher power is permitted.
The 27 MHz RCRS channels are not officially numbered by the FCC. R/C enthusiasts usually designate them by color, and fly different-colored flags from the antenna to show who is on which channel. On the other hand, some CB operators illegally use these channels for voice communications, and usually refer to them as an alternate of the closest voice channel below them (such as 3A, as shown in the table). In CB radios whose channel selector was a multiple-gang rotary switch that presented a binary number to the tuning electronics, it was a common illegal modification to add a toggle switch (or rewire an existing switch, such as the "PA" switch that coupled the microphone to an outside speaker) to modify the low-order bit and thus gain access to the R/C channels.
Because of interference from CB radios, legal or otherwise, the noise level, and the limited number of channels, most "serious" hobby radio-controlled models operate on other bands. Interference is especially important for model aircraft where it presents a safety issue.
The RCRS service has 50 channels just for model aircraft in the 72–73 MHz range, and 30 more channels for surface models such as cars and boats in the 75.4–76 MHz range. 0.75 watts is allowed on these numbered channels. Licensed amateur radio operators can use any amateur frequency for R/C, but those enthusiasts tend to use frequencies in their 6-meter band.
Part 15 devices.
Many toy R/C cars and wireless keyboards and mice operate on the 27 MHz R/C channels, especially 27.145 MHz. But most of these devices run far less than 4 watts and do not operate under the RCRS service. Instead, they operate under the FCC's Part 15 rules, which allow a wide variety of low powered devices to use the frequencies from 26.96 to 27.28 MHz, which covers CB Channels 1 through 27. Newer wireless keyboards and mice tend to use Bluetooth technology or frequencies in the 2.4 or 5.8 GHz region. Some R/C devices, especially quadcopters/drones, use cellular data techniques.
Some other of the R/C toys operate on the 49 MHz Part 15 channels, and often a pair of R/C cars will be sold with one on 27.145 and one on 49.860 to avoid interference. This allows less selective, and therefore less expensive, receivers to be used than if they were using channels in the same band.
In the days when CB required a license, some low-powered or toy walkie-talkies were exempt because they operated within Part 15. However, in 1976, the FCC phased in a shift of these 100 mW devices to the 49 MHz band, with operation on the CB frequencies to cease in 1983. More recently in the 1990s, low-powered handhelds using FM voice on the 27 MHz radio-control channels were also sold to operate legally under Part 15.
Broadband over Power Lines (BPL) technology uses a wide range of HF frequencies to transmit data (3.5 through 30 MHz), which includes the CB frequencies. There is great potential for interference, as power lines were never specifically designed to shield radio frequencies. RF leakage from BPL is regulated under Part 15 and is a big problem for amateur radio operators across all frequencies that the BPL uses.
ISM devices.
Another class of devices operating in the 27 MHz band are ISM (Industrial, Scientific and Medical) devices regulated by the FCC's Part 18 rules. Induction welding of plastics, and some types of diathermy machines commonly operate in this range. These devices centered on 27.12 MHz with a tolerance of ±163 kHz, that is, 26.957 to 27.283 MHz.
Adjacent frequency bands.
Adjacent frequencies are often used by illegal operators using modified CB or amateur radio equipment. Operators sometimes refer to this activity as freebanding.
The Business Radio Service has several channels just above the Citizen's Band, at 27.430, 27.450, 27.470, 27.490, 27.510, and 27.530 MHz, as well as above 29.710 MHz.
The federal government has the frequencies from 27.540 up to 28.000. Many civilian agencies use, or used to use, the frequencies 27.575 and 27.585 for low-power use. The US Coast Guard Auxiliary uses 27.980 MHz, it is similar to the Civil Air Patrol Frequency.
The 10 meter amateur radio (ham) band runs from 28.000 to 29.700 MHz. More business band channels are allocated between the top of the 10m band up to the 6-meter band (50-54 MHz), which is followed by television channels 2-6 (54-88 MHz), the FM broadcast band (88-108 MHz), and the airband (108-137 MHz).
Below the Citizen's Band, the U.S. military has the frequencies from 26.480 to 26.960 MHz. The Civil Air Patrol has 26.620 MHz, though it now uses mostly VHF frequencies. In the 1950s through the 1970s CAP volunteers with crystal-controlled CBs would put this frequency in their radios. Currently VHF military frequencies are more often used (the CAP is part of the US Air Force), as among other reasons, VHF radios are easier to acquire through military logistics than CB radios.
Below that is a broadcast auxiliary service band, the 11 meter shortwave band, and the 12-meter amateur band.