The term french door is defined as a door with glass applied vertically the length of the door. Therefore the word french in this case signifies glass, with the term apparently originating in France during the Renaissance. The french door was developed as a way to draw natural light from the outer rooms into the interior of the home.
The glass used in the manufacturing of a french door is either one large single panel (lite) or it is traditionally divided into several smaller panels (lites) with each lite being inserted separately into a mullion. A mullion is a vertical or horizontal strip used to separate the glass panes (lites). Standard true divided lite french doors are usually offered as single, five lite, ten lite and fifteen lite applications. Some manufacturers provide an imitation of a true divided lite french door by placing a one piece decorative grille on top of a single piece of glass.
Double prehung french door units are what comes to most peoples mind when they hear the term french doors, although french doors can be used in a variety of applications. In combination with the proper pocket door frame kit they can be used as pocket doors. A pocket door is a door that slides into and out of a hollow cavity, or pocket, in a doorway wall. When opened the door will disappear inside the space, hidden within the confines of the wall. French doors when applied with the right track and hardware can be utilized as a bi-fold door unit. Though bi-folds are most often thought of as closet doors, there are many beautiful applications for french bi-fold doors.
French doors continue to increase in popularity. Whether you intend to use them to separate a formal dining area, highlight the entrance to a library, or just to function as a room divider, french doors will not only serve their original intent, but also become a focal point of beauty in your home.