Most large buildings in New York have a pair of doors at their entrance. But, almost always, one of the two is locked closed, and you can only enter the building through the other one.
I haven't been able to figure out the reason for this phenomenon. I entertained the idea that fire codes might require two doors where the owner wanted only one, but that doesn't seem to make sense -- what good is a locked door in case of a fire? So, why keep building two doors at the entrance to large buildings, only to keep one perpetually locked? And, given that the two doors are there, what advantage does the owner gain from sealing one of them off? The practice is so common that I cannot believe that there isn't some good explanation for it, but does anyone know what it is?