As everyone who's ever done some plumbing knows, every fitting goes together clockwise. The more pipe you run, or the more complicated your geometry is, the more likely it is you'll end up with something you have to connect where you can't turn the ends the right way, or at all. There's a solution, of course: a union.
So the question is, what if you have that problem when you're running rigid conduit? I've never seen a union, and AFAIK using a union (or any ordinary plumbing fitting other than perhaps a coupling) is not kosher for conduit work.
But I've looked at complicated, professionally-installed rigid conduit runs where I can't imagine how they got it together conventionally. (It would be nice if I had some pictures, but I don't.) So how do they do it? Sometimes I can imagine that a left-hand thread would let you make something up tight, but I'm reasonably sure there's no such thing. But is there some kind of special conduit fitting that helps you assemble things where you can't turn them conventionally?