We get asked this question a lot. French doors come in a variety of configurations. One of the most common applications is to have one side of the door fixed and the other operable. The fixed door usually has a piece of wood trim [astragal] along the edge that the operable door can latch against. For the operable side you would order a privacy or passage knob and install it just like you would on any other door except the strike is installed on the edge of the inoperable door. The fixed inoperable door usually does not have a hole bored in it at all so you can install a dummy knob on each side of the door.
Also, the fixed door is usually held closed using a little latch on the edge of the door that slides upward into the jamb at the top, or downward into the threshold at the bottom. If these little slide latches are mounted on the edge of the door, they are called Flush bolts. If they are mounted on the surface, they are called surface bolts. You’ll want to measure the length of what you have so you can be sure to order the right size.
Materials Needed for French Doors
For the Operable Door – 1 Privacy, Passage or Entrance knob and Deadbolt. For the Fixed Door – 2 Dummy Knobs and two flush [or surface] bolts.
Enough Hinges for both doors.
What if you want handle sets on your french doors?
Exterior doors will often already have holes bored in both doors. This is fine for handle sets as the dummy handset for the inactive door will need the hole in the door to mount properly. In this case you would need one single or double cylinder handle set and one dummy handle set, flush bolts for the inactive door and enough hinges for both doors.