Although the MDI specification has been around since Windows 2.0, at that time MDI applications were difficult to write and required some very intricate programming work. Since Windows 3.0, however, much of that work has already been done for you. That support, with some enhancements from Windows 95, has been carried over into Windows 98 and Microsoft Windows NT.
The Elements of MDI
The main application window of an MDI program is conventional: it has a title bar, a menu, a sizing border, a system menu icon, and minimize/maximize/close buttons. The client area, however, is often called a "workspace" and is not directly used to display program output. This workspace contains zero or more child windows, each of which displays a document. ...