Lever action flush bolts are mortised into the door to create a flush surface either on the front of the door or in between two doors to securely bolt together
Generally when you have a set of double doors, one of the doors is inactive the majority of the time and the other door closes into it.
In other words, the hinges are on the outside and the doors close together in the middle.
The inactive door is held closed with a flush bolt at the top and bottom. Flush bolts are recessed into the door edge (top and bottom) and slide upward or downward into the jamb (or threshold) to keep the door closed.