Block programming or television block is a strategy of broadcast programming and radio programmers. Block programming can simply be defined as arranging programs on radio or television so that similar programs or programs of the same sort of genre are aired one after another. The concept is to provide similar programming to keep the viewers interested in watching. Radio stations use it consistently, by programming the same type of music for long periods of time. Notable examples of block programming was NBC's Thursday evening "Must See TV" lineup, which included two hours of sitcoms and one hour of ER, and Channel 4's "T4" program which often ran sitcoms like Friends back-to-back for an hour or more. This strategy is particularly common in cable television, where reruns are assembled into similar blocks to fill several hours of generally little-watched daytime periods. A particularly long program block, especially one that does not air on a regular schedule, is known as a marathon.
Block programming in radio also refers to programming content that appeals to various demographics in time blocks, usually corresponding to the top or bottom of the hour or the quarter-hour periods. For example, various musical genres might be featured; a country music hour; a three-hour afternoon block of jazz or a four-hour Saturday night '70s disco show.
Generally speaking block programming is anathema to modern competitive commercial radio, which traditionally uses uniform formats, other than a handful of specialty shows in off peak hours such as weekends (for instance, the infamous beaver hours in Canadian radio). The general rationale for not using block programming is that listeners expect a certain type of music when they tune into a radio station, and breaking from that format will turn those listeners away from the station; likewise, a station that airs its programming in hodgepodge blocks will have difficulty building listener loyalty, as listeners' music will only be on for a few hours of the day. This argument for homogenized radio was also a driving force behind the effective death of freeform radio in the late 20th century. The case of talk radio is indicative of the decline of block programming: prior to the 1980s, it was not uncommon to mix various blocks of talk programming together on one station, but this has declined dramatically in the late 1990s and beyond. A listener to a conservative talk radio station will have little interest in a progressive talk radio, sports radio or hot talk block, which reaches a different demographic; stations that have attempted the block strategy have historically been unsuccessful. Block programming of this nature is alive and well on outlets like public radio (such as NPR, BBC or the CBC) and in multicultural radio serving broad ethnic and cultural audiences, although even in this realm the idea of block programming is declining due to competition for donations.
Some program blocks have become so popular, that they have been transformed to full-fledged 24-hour channels. Current channels which started as program blocks include TeenNick (originally a program block on Nickelodeon); and Boomerang (which was once a program block on Cartoon Network). In addition, TV Land airs older shows that were once aired on sister channel Nickelodeon's Nick at Nite program block.