The FM Non-Duplication Rule was adopted by the U.S. Federal Communications Commission (FCC) on July 1, 1964, after a year's consideration. It limited holders of FM licenses in cities of more than 100,000 who also held AM licenses to simulcasting no more than 50 percent of their AM signal on the FM station. The commissioners considered the excessive simulcasting wasteful and an impediment to the development of FM broadcasting. A year later, the FCC reaffirmed the rule, and, after a delay requested by broadcasters, set its effective date for October 1965; some stations were granted exemptions if they could demonstrate their simulcasting served the public good.

Broadcasters generally resisted the rule at first, claiming it was overregulation that would impose considerable costs on stations for new personnel and equipment. It was challenged in court as a violation of the First Amendment and upheld in a decision written by future Supreme Court Chief Justice Warren Burger. Later the FCC extended the rule's requirements to 75 percent original content on FM stations and its applicability to stations in cities of more than 25,000.

The rule's implementation led to the rise of freeform radio in the late 1960s, where disc jockeys played whatever music they wanted without regard to genre or format, taking advantage of FM's higher sound quality and stereo capability. Most of their playlists tended toward rock of the era; in later years the canon of music and artists that developed evolved into the album-oriented and eventually classic rock formats. This growth often came at the expense of the classical music stations which had previously dominated the FM dial. By the late 1970s FM music stations had more listeners than their AM counterparts, even as they had largely moved away from their freeform roots, and in March 1986 the FCC repealed the rule to help AM stations, which had declined as FM grew.

Since the advent of commercial radio broadcasting in the United States during the 1920s, broadcasters and listeners had lamented the problem of static on its AM signals, usually created by interference from electrical storms. Engineers began to work on developing technology to transmit and receive higher-frequency signals above 40MHz over great distances. By the mid-1930s, Edwin Howard Armstrong had developed frequency modulation (FM) technology that could propagate signals widely with no static at all.

Several issues hindered FM's commercial establishment until the late 1940s. One was litigation over patents between Armstrong and RCA, Armstrong's former employer. RCA had chosen to invest heavily in television, another new technology. Regulatory disputes with the newly-created Federal Communications Commission and the intervention of World War II also played a role. Many FM stations soon had to shut down for lack of a market, despite the superior audio quality they offered, since listeners had to purchase FM-capable receivers. Armstrong's suicide in 1954 further adversely affected FM since he had financially supported Continental, one of the few FM networks at the time.

FM's fortunes began to turn in the late 1950s with television's rapid growth and AM radio's increasingly crowded dial. Popular music formats replaced scripted radio programming and variety shows migrated to TV. This left broadcasters looking for new directions to expand their markets. In 1957 the number of applications for FM licenses increased for the first time in almost a decade, primarily in smaller markets that had run out of space for new television or AM channels. In larger markets, the proliferation of AM stations had led the FCC to restrict many of the newer ones to daytime-only to avoid interference; since the FCC did not require that restriction for FM radio, it was a logical place for broadcasters to go.

The increasing popularity of stereophonic recordings also helped FM's growth. In 1952, New York City-area classical music station WQXR had hit on the idea of simulcasting on its AM and FM stations, with each carrying a different channel, so listeners could use two radios to create a stereo experience; other stations, mostly classical and jazz, followed the practice. Because the FCC frowned on that as a waste of spectrum, in 1961 it approved technical standards for wideband FM stereo broadcasting via the multiplexing capability of the higher frequencies and the first full-stereo wideband FM stations began broadcasting within months. Many existing FM stations still carried the same signal as their AM sister stations, assuming that FM was merely an added bonus for listeners who wanted the higher sound quality and were willing to pay for equipment to get it. Broadcasters at the time saw such little value in their FM stations that they gave advertising time away with purchases on their AM stations.

By 1963 the FCC was dominated by Kennedy administration appointees, led by chairman Newton Minow, who were interested in increasing competition in certain sectors of the radio market. That year the commission first discussed the possibility of restricting AM-FM duplication. While it had tolerated the practice initially since it allowed FM stations to get on the air, it now saw it as a waste of spectrum.