Generally, there are two types of user interface applications: SDI (single document interface) and MDI (multiple document interface). SDI applications have a single main form and enable us to view a single document at a time. Applications like Notepad, WordPad, Paint, and the Delphi Text Editor we built in the previous chapter are SDI applications.
Unlike SDI applications, MDI applications allow us to view more than one document at a time. An MDI application consists of a main (parent) form and none or many child window instances that are displayed within the parent form, as shown below.
Figure 17-1: An MDI image viewer
Besides the SDI and MDI interface styles, there are two other styles that are often used: the Explorer-style interface (see Figure 17-2) and the tabbed interface (see Figure 17-3). Both of these styles are actually SDI interfaces built with complex components like TTreeView and TPageControl.
Figure 17-2: The Explorer-style interface
Figure 17-3: The tabbed interface