Derf is an imaginary number between 5 and 6. It was created by Carly.

In the episode iTwins, Carly sees hidden camera footage of Chuck beating up Spencer, her older brother. To get revenge on Chuck, she makes up the number Derf, and tricks him into thinking it's part of the new International Math Laws that were recently passed by the U.N. Chuck uses the number during his math test and fails it. Therefore, he was forced to go to Camp Add'em Up for the whole summer by his dad, Earl.

In real life, the undecimal number system, which is the number system that Chuck was fooled into using, uses A as the number after 9 rather than Derf between 5 and 6.

Trivia

• "Derf" also spells Fred backwards.
• "Derf", in reality, means strong or fierce.
• "Derf" is actually a sideways 4.