MultipleDocumentInterface is a Windows term for an application in which several documents can be open at a time within sub-windows of the main application window.

An alternative would be a SingleDocumentInterface, in which each application window can contain only one document, but there can be multiple application windows open on the screen to display several documents at once.

Note that the Macintosh interface (OS 9 and OS X) is neither Single''''''Document nor Multiple''''''Document; rather, it's a sort of blend of both, because there is only one menu bar for the entire application but each document window is independent.

MultipleDocumentInterface is often preferable because:

	* The application can provide ways of arranging the document sub-windows to make it more convenient to view them or exchange data between them. 
	* The application can let the user perform operations (e.g., search & replace) on multiple files simultaneously.
	* There is a single menu bar for the entire application. The document windows typically do not have their own menu bars, saving space when there are multiple documents visible.

MultipleDocumentInterface is often '''not''' preferable because:

	* Though the application window makes it more convenient to exchange data between documents within the same application, it makes it more difficult to exchange data between documents in different applications.
	* Resizing a multiple-document window takes several steps: first you have to resize the application window, then you have to move and resize the document sub-windows within the application window.
	* When you need to resize the application window, you often end up obscuring some of the document sub-windows. Often, the application window puts up a scroll bar to deal with this, but that means there is an application-window scroll bar as well as a scroll bar for each document window -- a situation that is confusing for newbies and inconvenient even for experienced users.
	* When a document window is zoomed to full size, Windows often blends its title bar with that of the application window, resulting in a confusing situation in which there are two sets of window controls on the same window -- the application's and the document's. Many newbies don't know how do distinguish the two.

MS encouraged developers to use MultipleDocumentInterface, then they told developers to use SingleDocumentInterface because MDI was too confusing; now I think they're back to MDI again.