Windows MDI provides applications with a standard interface for displaying multiple documents within the same instance of an application. An MDI application creates a frame window that contains a client window in place of its client area. An application creates an MDI client window by calling CreateWindow with the class MDICLIENT and passing a CLIENTCREATESTRUCT structure as the
function's lpvParam parameter. This client window in turn can own multiple child windows, each of which displays a separate document. An MDI application controls these child windows by sending messages to its client window.
For more information about MDI, see the Microsoft Windows Guide to Programming.