General Order 40

This order grouped the AM radio band transmitting frequencies into three main categories, which became known as Clear Channel, Regional, and Local.

The first formal regulations governing broadcasts intended for the general public were adopted effective December 1, 1921.

[1] The number of broadcasting stations grew dramatically in 1922, reaching over 500 by the end of the year, and the government began making available additional frequencies.

The Commission also reallocated frequency assignments to reduce interference and provide better service to smaller communities and underserved rural areas.

These issues were especially important at night, when a change in the ionosphere meant that radio signals traveled much greater distances.

Twelve years later this omission was addressed by the North American Regional Broadcasting Agreement (NARBA), which took effect March 29, 1941.

General Order 40's implementation took effect at 3 a.m. Eastern time on November 11, 1928, and required most U.S. radio stations to move to a new broadcast frequency.

[5] A survey conducted by Radio Retailing magazine concluded that, despite some continuing issues, the changes meant that "on the whole, the broadcasting situation has been much improved".