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 provided by the system.
Note MDI is an application-oriented model. Many new and intermediate users find it difficult to learn to use MDI applications. Therefore, many applications are switching to a document-oriented model. Therefore, you may want to consider other models for your user interface. However, you can use MDI for applications which do not easily fit into an existing model until a more suitable model is introduced.