A Diploma is what one receives if one finishes high school, which Rigby didn't do. Some may consider one "not smart" if one doesn't have one, such as Mordecai. Also, the Burger Bros will sometimes have a special day where everyone with a high school diploma will get free hamburgers. Everyone got a hamburger but Rigby, because Rigby didn't have one. There is also a website called Education Mentor with easy questions where you can receive a diploma.
- Rigby can't spell diploma, as he wrote "diplowma" on his fake diploma.
- Mordecai and Rigby were friends in Junior High, but Rigby didn't finish High School, but it is possible that he either dropped out and had a GED or he has been kicked out, so they obviously remained friends.
- Sensai has a diploma for the City Community College in Death Punchies.