How to Put iPad in Landscape Mode

I am an expert computer user, having written many software titles for OSx as well as PC and Linux and Unix. But I am finding the iPad exceedingly difficult to learn.

I can muddle through the basic things, but there is one thing I have not figured out how to do, and after searching the Internet and this forum, I still can't find out how to do it.

How do I put the iPad in landscape mode? I feel like such an idiot because I have tried to figure out how to put it in landscape mode for hours, and can't find the option anywhere. I can't believe I'm the only person who can't figure this out.

Thanks for the help!