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. But many others soon followed.