shave and a haircut,

.... two bits!

3 Common rhythmic patterns in American popular music during the late 19th Century/early 20th Century:

1. "Shave and a Haircut"
2. The Charleston Rhythm
3. The Bo Diddley beat (ok, this one is more like almost mid-century, meh.)

"Shave and a Haircut, Two Bits" rhythm can be heard at the very end.

The Charleston

Bo Diddley beat, as used by The Strangeloves in 1965 (it was created nearly 20 years earlier)

And... whoa, somebody's created 99 piano variations on the "shave and a haircut" melodic pattern.  

Why?  

I'm not really sure.