When is the use of "north" more appropriate than "northern" and vice versa?

North, South, East, West, can be used as adjectives, but so can Northern, Southern, Eastern, Western. What rules, if any, govern which is appropriate when?

North can also be used as a noun: one of the points of the compass is called north, and "the north" is frequently used to refer to geographic regions. Northern is always an adjective (notwithstanding its occasional use as a proper noun, as in "I attended college at Northern").

If you had to choose one of them to use as an adjective, you would probably want to use northern. North as an adjective is generally limited to proper nouns or idiomatic/traditional usage: the North Pole, the North Star, a north wind, the north face of the mountain, the north end of town.