Many of the older Oxford and Cambridge colleges have two doors in one doorway: the external one for the 'interruptible/not' signal, and the internal one to keep the heat in when the outer is open. We have a term for it: 'sporting one's oak'.

http://www.grammarphobia.com/blog/2008/02/sporting-the-oak.h...

This was a great system in medieval buildings where most doors opened directly to the outdoors, which in itself is a great privacy feature.

Except for when someone doesn't use that model. I work with people who keep their door closed all the time, but they're not in a "don't bother me" mood.

Then they suffer the concequences of people not going up to them. That, or they want people "to know" that the door closed means they can come in - those who they haven't told will shy away. Its kinda a way of filtering, or they're socially inept.