Originally Posted by: jedftSo, is there a way to communicate which way a particular tune is going to go, say maybe if someone is sitting in with a band?

Sure! Tell them the key (D-sharp or whatever), then specify to them to play the IV & V higher or lower than the I.

In many case it does sound just as good. And some people in an open jam won't know what you are talking about anyway. You'll be lucky if they know enough to know the IV & V of whatever key you pick. :p

And sometimes it helps to have two instruments doing the same chords in different directions. Makes for a bit of variety, contrary motion in the music that makes it sounds fuller and richer.

Notice that in many blues turnarounds the bass and guitar go in opposite directions to both wind up on the V chord.

Like this:

http://www.guitartricks.com/lesson.php?input=10307&s_id=274