They are made of different materials, are intended for different uses, and move using different movement systems. One of the more interesting of the movement systems happens to be a current growing trend called barn door hardware, also known as flat track hardware. Even though that is this author's preferred style of door hardware, that is neither here nor there. The design of this article is to address the construction of doors and their subsequent anatomy.
As stated previously, doors can come in all shapes and sizes. Here we'll talk about the make-up of a standard 3'x7' door, the door most commonly used in most homes. This description will be given as though one were looking at a door in the closed position.
Starting off, there are two long vertical pieces that run the length of the door on both the left and right side. These are called the stiles. On the top and bottom there are two horizontal pieces that lock into the vertical stiles; these are called the top and bottom rail. There are also two other horizontal rails, one towards the top, and another that sits at the height of the door knob or handle. The one near the top is called the intermediate or cross rail, and the one near the center is called the lock rail. Running vertically in the center of the door you have what are called mullions. These parallel the side stiles and lock into the horizontal rails just described.
Between all of these rails, stiles, and mullions you'll notice that there are six panels- four large and two small. The two smaller ones are almost square and sit at the top of the door.