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

The star of the FX series also admits he was initially hesitant about the crossover concept as 'It's Always Sunny in Philadelphia' heads into its 17th season.

This week's "Abbott Elementary" episode is just half of the story. "Volunteers" featured the long-awaited crossover with FX's long-running "It's Always Sunny in Philadelphia," as told from the vantage point of the hit ABC sitcom.