Which of these states do you consider Southern?

[President Punxsutawney Phil]

"Appalachia" really deserves it's own distinction from "The South".    The two really do have quite a few differences.

Where is the line between the two, though? Is Tennessee "Southern" or "Appalachian"? Should Pennsylvania be considered part of "Appalachia"?

If you use this distinction, the only state that would purely fit into Appalachia is West Virginia, but parts of Pennsylvania, Virginia, Kentucky, Tennessee, North Carolina, and Georgia would fit as well.  But, by the time you move far enough down the mountain chain to Tennessee (look at places like Knoxville and Chattanooga), the culture is much more Southern than Appalachian.  Plus, over two-thirds of Tennessee lies outside the Appalachian Mountains (even Knoxville isn't really high up at all).  Where I am in Middle Tennessee, it is pretty flat, and parts of West Tennessee have more in common with Mississippi than East Tennessee.

With this in mind, unless you are getting very specific with your divisions "Appalachia" should just be considered part of "The South", IMO.

Amen.  Do people think regions can't be diverse in and of themselves?  The South, for some reason, seems to get more of this attention than other areas.  Nobody denies that both Ohio and North Dakota are Midwestern, for example, though they are clearly not exactly the same culturally.  Regions, however you chose to define them, should be static (once they're defined), and it's perfectly fine if they encompass several different types of people.