The Multiple Document Interface (MDI) is a specification for applications that handle documents in Microsoft Windows. The specification describes a window structure and user interface that allow the user to work with multiple documents within a single application (such as text documents in a word processing program or spreadsheets in a spreadsheet program). Simply put, just as Windows maintains multiple application windows within a single screen, an MDI application maintains multiple document windows within a single client area. The first MDI application for Windows was the first Windows version of Microsoft Excel. Both the Program Manager and File Manager in Windows 3 are MDI applications.
Although the MDI specificiation has been around since Windows 2, at that time MDI applications were difficult to write and required some very intricate programming work. With Windows 3, however, much of that work has already been done for you. Windows 3 includes one new window class, four new functions, two new data structures, and eleven new messages for the specific purpose of simplifying MDI applications.