High Quality Content by WIKIPEDIA articles! Graphical computer applications with a multiple document interface (MDI) are those whose windows reside under a single parent window (usually except for modal windows), as opposed to all windows being separate from each other (single document interface). Such systems often allow child windows to embed other windows inside them as well, creating complex nested hierarchies. In the usability community, there has been much debate about which interface type is preferable. Generally, SDI is seen as more useful in cases where users work with more than one application. Software companies have used both interfaces with mixed responses. For example, Microsoft changed its Office applications from SDI to MDI mode and then back to SDI, although the degree of implementation varies from one component to another.