I know from other threads that some of the dopers here were soldiers in the American allied forces in West Berlin, so I think there should be some first hand experts here to know.
When Berlin was split into a capitalist Western half and the communist Eastern one, divided by the iron curtain, the allied forces still had and used their rights to patrol in the entire city. One of the big crises of the Cold War was when the Soviets denied Americans troops free access to the Eastern part, and the Americans reacted by deploying troops along the border. IIRC, the Russians backed down and agreed to grant US troops free access to East Berlin without Russian monitoring. Right so far?
If yes, I don’t quite understand what Checkpoint Charlie was intended for. I imagine Soviet/East German troops would have been standing there, checking documents of military vehicles heading towards East Berlin. But this would have been a violation of the free access thing. OTOH, it would hardly work without any document check since non-military vehicles would not have had the privilege to use Checkpoint Charlie. I vaguely remember that they found a weird procedure that provided enough control to pacify the Russians and, at the same time, little enough control to make the Americans agree with it. Anybody knows how exactly the clearance procedure at the checkpoint looked like?
Related question: Did the Soviets have the right to patrol in West Berlin? Did they do so on a regular basis? I suppose they would not have been very welcome.