Checkpoint Charlie was the third crossing point in the military alphabet: Alpha as you entered East Germany, Bravo as you entered West Berlin, and Charlie between West and East Berlin.

It was reserved for Allies and non-Germans. Americans could use it. West Germans could not. Fourteen crossing points existed in total: eight between East and West Berlin, six more between West Berlin and East Germany.

The booth standing today is a copy of the original from 1961. After 1989, everything was removed. The portrait on the booth shows Jeffrey Harper, stationed in Berlin in 1989, one of the last US soldiers before they withdrew completely.

In 1961, months after the Wall went up, Soviet and Allied tanks faced each other here. The world held its breath. The dispute was silly: Eastern guards had started asking for papers, violating the free passage agreement. After 16 hours, the tanks withdrew. One false move could have triggered the Third World War.

VoiceMap's self-guided audio tours explain the military alphabet naming system, recount the 1961 tank standoff, and reveal how 5,000 people escaped.