MDI (Multiple Document Interface) is a common approach for an application's structure. An MDI application is made up of several forms that appear inside a single main form. If you use Windows Notepad, you can open only one text document, because Notepad isn't an MDI application. But with your favorite word processor, you can probably open several different documents, each in its own child window, because the word processor is an MDI application. All these document windows are usually held by a frame, or application, window.
Increasingly, Microsoft is departing from the MDI model stressed in Windows 3 days. Even recent versions of Office tend to use a specific main window for every document: the classic SDI (Single Document Interface) approach. However, MDI isn't dead and can sometimes be a useful structure, as demonstrated by browsers like Opera and Mozilla.
The MDI structure gives programmers several benefits automatically. For example, Windows handles a list of the child windows in one of an MDI application's pull-down menus, and specific Delphi methods activate the corresponding MDI functionality to tile or cascade the child windows. The following is the technical structure of an MDI application in Windows:
The main window of the application acts as a frame or a container.
A special window, known as the MDI client, covers the whole client area of the frame window. This MDI client is one of the Windows predefined controls, just like an edit box or a list box. The MDI client window lacks any specific user-interface element, but it is visible. You can change the standard system color of the MDI work area (called the Application Background) in the Appearance page of the Display Properties dialog box in Windows.
There are multiple child windows, of the same kind or of different kinds. These child windows are not placed in the frame window directly, but each is defined as a child of the MDI client window, which in turn is a child of the frame window.