In true double French doors, each of the little windows 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 real double French door.
Double 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 exterior Double French Doors with a single piece of glass in each door, and a latticework over it to make it appear as many individuals little windows.
One concern with using Double French Doors as patio doors 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.
Double French doors are also sometimes known as French windows, which hint at the history of this beautiful architectural feature. 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.