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.