Pre-broadcasting frequency allocation rules
The first law which governed assignment of frequencies to radio transmitting stations was enacted in 1912. It divided the radio spectrum into four portions:
- A government band from 600 to 1600 meters (500 to 187.5 khz) chosen for its groundwave coverage.
- Two commercial-use bands:
- Above 1600 meters (below 187.5 khz (which had even better groundwave coverage, and was used for transoceanic communication facilities).
- From 600 meters to 200 meters (500 khz to 1500 khz) used by commercial stations which had less need for long-distance coverage. Some frequencies in this range, such as 300 meters (1000 khz) and 220 meters (1365 khz), were set aside because ship antennas were too short for effective use on longer wavelengths.
- An amateur frequency, 200 meters (1500 khz), which was thought to be useless and was grudgingly given to amateurs so they were not totally frozen out.
Individual amateurs were permitted to be granted special licenses for use in the 200-600 meter frequency band to permit communications between them for "relay" work, but they were highly restricted. The rules stated that "...a special license will be granted only if some substantial benefit to the art or to commerce apart from individual amusement seems probable."
The rise of radiotelephony
Initially, all radio transmissions used the telegraphic (Morse) code, but as spark transmitters were phased out and vacuum tubes replaced them, telephonic (voice) communication became practical. In the latter part of 1919, wartime restrictions on radio transmission ended, and many commercial, experimental, government, and amateur stations began to experiment with the new vacuum tube-based transmitters, so that by September 1920, voice and music transmissions could be heard almost every evening on the air, as reported in a contemporary amateur radio magazine, QST.
While it is known that on November 2, 1920 a Westinghouse broadcast from East Pittsburgh, Pennsylvania was made (by a station that developed into KDKA, but at the time using a Special Amateur license as 8ZZ), the frequency is not certain. It was impossible to use the low frequencies reserved for Government usage, so it was necessarily in the 500 khz to 1500 khz range, but reports differ on the frequency, possibly 550 meters (545 kilohertz) as cited in contemporary reports, or 330 meters (909 khz), given in later publicity by KDKA. Westinghouse stations are also reported to have broadcasted on 375 meters (800 khz). By negotiation with the Commerce Department, Westinghouse was able to get a licensing agreement which became general for other broadcasters, with a specific wavelength (corresponding to a specific frequency) specified; the first such license granted was to WBZ (then in Springfield, Massachusetts), on September 15, 1921, and other licenses soon followed.
Before opening the band: Limited frequencies
Initially, in 1921, all AM broadcasting stations were given a wavelength of 360 m, corresponding to a frequency of 833 khz. This of course would mean that no two stations in the same area could be on at the same time. It should be noted that Westinghouse had originally expected to be assigned this wavelength for its own stations, but had not intended this to be the general broadcasting frequency, although the Commerce Department decided to make it such.
In December 1921, a second wavelength, 485 m (corresponding to a frequency of 619 khz) was assigned for some special types of broadcasts like weather reports and farm information. But rather than assigning some stations to one frequency and some to the other, the division was made by content, so a station would switch frequencies when it was time to give a weather report.
In September 1922, a new entertainment frequency of 750 khz was added. In theory, 750 khz was assigned to better quality, higher powered stations. These stations were designated "Class B" outlets, while those on 833 khz became known as "Class A" stations. About thirty stations nationwide would eventually qualify to use 750 khz.
The broadcast band opens up
By early 1923, it became obvious that two entertainment frequencies were not going to be enough, and a conference was called by the Secretary of Commerce (later to be President), Herbert Hoover. The conference was held from March 20 to 24 of that year, and issued a report recommending an expansion of the band. These recommendations were used as a basis for the new frequency reassignments, and on May 15, 1923, the government started assigning different frequencies to different stations, at 10khz intervals. Initially, these went only from 550 to 1360 kHz. The frequencies from 550 to 1040 kHz were set aside for Class B stations, while the remaining frequencies were designated for Class A. Not all stations left the 833 kHz frequency at this time, however, and these stations were designated as Class C. (For stations that did change frequency on that date, see 1923 frequency reassignments).
In April 1925, the band was further expanded to 550-1500 khz, providing 15 additional Class A frequencies. In June 1926, however, the Attorney General issued an opinion that the Commerce Department, under the 1912 act, had no power to regulate frequencies of stations, and a number of stations moved to frequencies which they wished to use, regardless of interference to other stations, even taking split channels (frequencies that were not exact multiples of 10 khz). In 1927 the Congress enacted a new Radio Act, creating the Federal Radio Commission, and gave it power to make frequency assignments.
On June 15, 1927, nearly 600 stations (out of 694 nationwide) were moved to different frequencies. (See 1927 frequency reassignments.) Another big "realignment" took place on November 11, 1928, in an attempt to reduce interference. the FRC shifted many stations, and deleted others. (See 1928 frequency reassignments.)
In late 1939, a treaty (the North American Radio Broadcasting Agreement, usually referred to as NARBA) was signed by the United States, Canada, and Mexico, establishing some clear channels for each country, and as a consequence those frequencies were to be vacated in the other signatory countries. It also provided for expansion of the broadcast band to 550-1600 khz. These changes were originally planned to take effect on July 1, 1940, but the date for the changes was postponed to March 29, 1941. (See 1941 frequency reassignments and  for some more details.)
In the early 1950s, the band was extended to 540-1600 khz. The first 540s appeared in 1954 or 1955.
In 1995, the FCC again extended the standard broadcast band to 540-1700 kHz.