The two sitcoms could hardly be more tonally different, but this week’s crossover episode of “Abbott” was relatively seamless.

The highly anticipated crossover episode of “Abbott Elementary” and “It’s Always Sunny in Philadelphia” blends the raunchy, NSFW style of the FX show with the witty, family-friendly romp.

Here's some background and inside jokes to help fill in some of the gaps for viewers who don't watch both Philly-based shows.