The multiple document interface (MDI) is a specification that defines a user interface for applications that enable the user to work with more than one document at the same time.
This overview describes the structure of an MDI application and how to take advantage of the built-in MDI support found in the Microsoft® Win32® application program interface (API).
MDI is an application-oriented model. Many new and intermediate users find it difficult to learn to use MDI applications. In the future, applications will use a more document-oriented model. Therefore, you may want to consider models such as workspaces, or workbooks for your user interface. However, you can use MDI for applications which do not easily fit into another model until a more suitable model is introduced. For more information on workspaces and workbooks, see The Windows Interface Guidelines for Software Design.