What is Multiple Document Interface (MDI)? In most VB.NET applications, it is using a single document interface (SDI). In this type of interface, every window is unique to aother window. But in multiple document interface, it works by having one parent window with child windows under it. See the screenshot below: As you can see, there is one parent window (in gray color) and there are 3 child windows (in blue, violet and orange color). You can have more than 3 child windows depending on your application requirements. But you can only have one parent window. Depending on the design of your MDI...