Astragals cover the gap between a pair of doors. Usually they are fixed to an inactive door, with a flush bolt mortised at the top to keep the door stationary when the active door is opened. When the active door is closed, the astragal stops and lines up the active door with the inactive door.
Interior Astragals are completely symmetrical and only need one flush bolt at the top of the astragal. In order to open the inactive door, the flush bolt needs to disengaged.
Exterior Astragals are asymmetrical to accommodate the weather strip needed for the active door. Exterior doors also need a flush bolt at the top and the bottom of the astragal. The inactive door can only be opened after the active door is opened and both flush bolts are disengaged.