A French door  is a door that has multiple windows  set into it for the full length of the door, which open outwards. Traditional French doors are assembled from individual small pieces of glass and mullions These doors are also known as true divided lite French doors. French doors made of double-pane glass (on exterior doors for insulation reasons) may have a decorative grille embedded between the panes, or may also be true divided lite French doors. The decorative grille may also be superimposed on top of single pane of glass in the door.