French doors originated in France during the 17th century. The Renaissance period was well advanced, and art and architecture flourished under the influence of new ideas. The doors originated as windows that reached to the floor and led onto small balconies. By the end of the 17th century, the doors were starting to appear in England as well.
During the early years of the 16th century, France was at war with Italy, which had been experiencing the Renaissance for nearly a century at that point. After their victories, the French brought home some of the artwork they came across as well as some architectural ideas. The Renaissance style focuses on proportion, symmetry, regularity and geometry, reverting back to the style of ancient Roman architecture. Emphasis was placed on light, which lead to a greater incorporation of windows in ways they hadn't been used before--including their use as doors.
The French doors had a purpose beyond style. The windows in the door allowed more light to enter a room. This was important in a time before the discovery of electricity, as it allowed people to have light in their homes for a longer part of the day. Light could be admitted to hallways and interior rooms that had no other windows.
Doors were originally made of wood, with wrought iron to add style and structure. As the glass industry developed, the French began to embed windows in the structure to allow the light to come in. French doors were traditionally made up of individual glass windows, or lights, held by mullions, which were the bars of wood creating frames for the windows. There is no limit to the number of lights that each door can have. In many doors today one double pane of glass is covered by a lattice to simulate multiple windows. The door itself today can be made of many materials, though most commonly wood or rigid PCV and aluminum are preferred.
As time passed, these doors have become more elaborate. Casement windows that grew to meet the floor, originally found only on upper stories leading to balconies, French windows are now included in homes and office buildings, giving access to courtyards and gardens. French doors are a choice of architects and builders who want to create an atmosphere of spaciousness with a sense of history.