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But “The Simpsons” was locked into an unusual — and now vintage — deal. The show was first sold into syndication in 1993. While an enormous hit for Fox, “The Simpsons” always stood out because it was animated. When Fox tried to place live-action comedies adjacent to it, they never really worked.

So the stations paying hefty rights fees insisted on maintaining exclusivity — meaning no sale to a cable network for as long as they were buying new seasons of reruns. “The Simpsons” — with a cast that never visibly aged — kept making new episodes on Fox, and the syndication contracts kept going.

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– http://www.nytimes.com/2013/11/16/business/media/simpsons-reruns-sold-to-fxx-in-first-cable-deal.html?ref=todayspaper