A French door is a door set with many small window panes. The most common manifestation, a double French door, is made up of two French doors set on different sides of the same door jamb, so that they open away from one another.
In a true double French door, each of the little windows, or lites, is a separate piece of glass. These are divided by wooden bars called mullions, which hold the glass in place. However, many cheap modern imitations save labor and cut costs by making each door’s many windows out of one single, large piece of glass, with a plastic or wooden latticework that is fastened to the glass to make it look like a real double French door.
A double French door can be made up of either doors that swing on pin hinges, like most doors, or they can be what is known as pocket doors: doors that slide into narrow spaces, or pockets, in the wall, making them all but disappear when they are open. A pocket style double French door is traditionally used as an interior door, while double doors that swing on pin hinges may be either interior or exterior doors.
French doors are becoming common and quite popular choices for patio doors, replacing sliding glass patio doors, especially in upscale or custom built homes. These doors typically have double-paned glass for better insulation, as it has a much lower rate of heat transfer than single-paned glass. Because of this, it is much more practical to make an exterior double French door with a single piece of glass in each door, and a latticework over it to make it appear as many individuals lites.
One concern with using a double French door as a patio door is that they make it easier for someone to break in, because both doors pivot and can be more easily forced open. Modern enhancements have therefore introduced three-point locking, where one of the doors has a slide lock that secures it to the floor and the top of the door jamb as well as a locking knob. Any time you install French doors as an exterior door, it is important to ensure that they have three-point locking.
A double French door is also sometimes known as French windows, which hints at the history of this beautiful architectural feature. In the late 1600s in France, tall double casement windows became popular. Since these went all the way down to the floor, they were developed into doors, normally used on private balconies. These graceful double doors were also incorporated into the architecture for plantation homes in the southern United States, where they helped to keep homes cool by promoting better air circulation when open.