My girlfriend, who is not a native English speaker, asked me this yesterday: why are some geographical regions prefixed with the nouns North, South, East and West, but others are prefixed with the adjectives Northern, Southern, Eastern and Western?
For example, it's far more common (to my British ear at least) to hear people talking about "Eastern Europe" and "Southeast Asia" than the alternatives "East Europe" and "Southeastern Asia."
Why is this the case? I couldn't think of a convincing explanation. Is there a hidden logic that I can't fathom, or is it just one of those wierd idiomatic things?