What do the window and door sizes represent?

Each set of construction drawings uses a generic window and door sizing system that does not apply to any particular name brand. For example, a common window size on our plans is 4050. This is a window that is 4'-0" wide by 5'-0" tall. In like manner, a common door size is 2668, which means the door is 2'-6" wide by 6'-8" tall.