At my day job all the different sections of the building have large heavy wooden double doors wherever a large room meets a hallway. I think they are there to help the heating/air conditioning work properly and to slow down the spread of a potential fire. All the doors have an engraved sign permanently affixed to them saying “Please keep this door closed at all times.” If you look at that sign and drop a line straight down to the floor you will see the little wedge of wood that is always propping it open.