The Abbott Elementary & It's Always Sunny In Philadelphia crossover had hilarious character interactions, but it hid callbacks in its witty writing.

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.

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.