http://www.speakdanish.dk/en/grammar/adverb-dynamic-static.php This applies to other adverbs too, like ud/ude, op/oppe, ned/nede, etc.
Wow, this thread got kind of heated! Okay, here is a (I hope) less controversial question of clarification. Is there a contextual difference between using "Hvor er du" and "Hvor er du henne?" Does the meaning of the question change subtly by adding "henne?" I am hoping a native speaker will weigh in on this. Tak for dit svar.