The United States has a decentralized, market-oriented television system. Unlike many other countries, the United States has no national broadcast programming services. Instead, local media markets have their own television stations, which may be affiliated or owned and operated by a TV network. Stations may sign affiliation agreements with one of the national networks. Except in very small markets with few stations, affiliation agreements are usually exclusive: If a station is an NBC affiliate, the station would not air programs from ABC, CBS or other networks.However, to ensure local presences in television broadcasting, federal law restricts the amount of network programming local stations can run. Until the 1970s and '80s, local stations supplemented network programming with a good deal of their own produced shows. Today, however, many stations produce only local news shows. They fill the rest of their schedule with syndicated shows, or material produced independently and sold to individual stations in each local market.
The major commercial television networks in the U.S. are NBC and CBS, which date to the early days of television (in fact, they both began in the 1920s as radio networks), and ABC, founded, also as a radio network, in 1943. In big cities, affiliates of these networks almost always broadcast in the VHF band, which, in the days before cable became widespread, was premium real estate.
Major-network affiliates run very similar schedules. Typically, they begin weekdays with an early-morning locally produced news show, followed by a network morning show, such as NBC's Today, which mixes news, weather, interviews and music. Syndicated programming, especially talk shows, fill the late morning, followed often by local news at noon (Eastern Time). Soap operas dominate the early afternoon, while syndicated talk shows such as The Oprah Winfrey Show appear in the late afternoon. Local news comes on again in the early evening, followed by the national network's news program at 6:30 or 5:30 p.m., followed, in the Central Time zone, by more news.