Why did the Saturn V's first two stages have splines on the body?

Here's an image for illustration

The splines I'm talking about wrap around the lower section of this booster stage. It's smooth around the section containing the USA logo then splines wrap around the section just above that and then the next section above is smooth again.

I'm guessing this serves and aerodynamic function but that's only a guess. They also are on the tapered part at the top of the 2nd stage.

Any resources out there explaining these?

Submitted March 08, 2016 at 11:46AM by trevize1138 http://ift.tt/1Xa8MHx