Windows multiple document interface (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 which contains a client window in place of its client area. The application creates an MDI client window by calling CreateWindow with the MDICLIENT class and passing a CLIENTCREATESTRUCT data structure as the function's lpParam 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. The following briefly describes these MDI messages:
| Message | Description | |
| WM_MDIACTIVATE | Activates a child window. | |
| WM_MDICASCADE | Arranges child windows in a cascade format. | |
| WM_MDICREATE | Creates a child window. | |
| WM_MDIDESTROY | Closes a child window. | |
| Message | Description | |
| WM_MDIGETACTIVE | Returns the current active MDI child window. | |
| WM_MDIICONARRANGE | Arranges all minimized child windows. | |
| WM_MDIMAXIMIZE | Maximizes an MDI child window. | |
| WM_MDINEXT | Activates the next child window. | |
| WM_MDIRESTORE | Restores a child window from a maximized or minimized state. | |
| WM_MDISETMENU | Replaces the menu of an MDI frame window, the Window pop-up menu, or both. | |
| WM_MDITILE | Arranges all child windows in a tiled format. |