Spoilers for the Abbott Elementary and It’s Always Sunny in Philadelphia crossover are ahead! If you haven’t watched the first half of this event yet, you can stream the Abbott episode with a Hulu subscription.

The 'Abbott Elementary' EPs talk jokes cut from 'Always Sunny' crossover, including backstories for Frank and Mr. Johnson.

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.