Why do we have 20 Dollar Bills?

It just occured to me...why do we have 20 dollar bills? I mean, I understand 1, and 5, and 10, but then why 20? And why is the next bill a 50? These numbers are all multiples of 5, in this order: 0, 1, 2, 4, 10...Perhaps the treasury knew that they wanted 10 and 50 dollar bills, and wanted something in between. But that would be a 30 dollar bill. Or, if they wanted a bill between 0 and 50, they'd need a 25 dollar bill. However, a 25 dollar bill would be difficult to give change from, since 25 is harder to subtract from than 20. But in that case, then why do people create such obscurely priced merchandise?

My pizza cost $2.40.

Curiosity: I was on the phone so long yesterday that my hand was cramped up in the phone-holding position (hand up to ear).