The shelf life of your favorite show is often extended by syndication, but how much do you know about this niche of the TV world?

All TV programs eventually get cancelled. (Except for The Simpsons.) Reruns of the longest lasting can air for decades after their initial runs, typically if they banked at least 100 episodes. This is called “off-network syndication” and it refers to shows that originally aired on a major network.

There’s also something called “first-run syndication,” which describes stuff like game shows that are produced fresh and sold directly to local stations. Almost since the dawn of television, TV stations have relied on syndicators to fill spots in their programming during non-primetime hours. In the 1950s, Ziv Television Programs pioneered the idea of first-run syndication with the 1950-56 show The Cisco Kid. Even though TV wasn’t yet broadcast in color, Cisco was filmed in color. When the rest of the country adopted color TV, Cisco reruns aired into the 1970s.

Years later, shows that were cancelled by networks actually went the opposite route. Silver Spoons, Charles in Charge, and Too Close For Comfort lived on with new episodes in syndication. But those shows’ popularity paled in comparison to the two biggest ever hits in first-run syndication: Star Trek: The Next Generation and Baywatch. The first season of the latter was a huge flop on NBC back in 1989, but in syndication it became the most watched show on the planet and ran for 11 years.

But let’s not forget about off-network syndication, which can also be a big money earner. For the syndication rights to ER, TNT paid $1.2 million per episode. The USA Network, meanwhile, paid about $750,000 to air reruns of Walker: Texas Ranger. Why cough up all of that money? Because the more popular the show, the higher its potential ad revenue, especially when networks edit or even speed up reruns to fit in more commercials. And even at a million bucks an episode, it’s still cheaper than producing new shows.