The Multiple Document Interface(MDI) is a programming interface under Windows, where the user can open and edit multiple documents on the screen simultaneously. In contrast, with the Single Document Interface( SDI) only one document can be opened and managed in one window.
In Multiple Document Interface, a main window is opened for the program and in this main window, multiple windows can be opened for the individual documents running under different application programs. For example, documents with spreadsheet, word processing or graphics programs can be displayed and processed separately. The document windows can be closed individually without closing the main window. The program is only closed when the main window is closed.
The MDI interface is used in Windows to link several objects together. Besides the Single Document Interface and the Multiple Document Interface there is also the Tabbed Document Interface where sub-windows can be opened within a main window. This technique is increasingly used in web browsers.