Modular Windows desktop applications need a well-defined architecture. Modularity has to be supported throughout the whole application and its architecture. This implies a modular UI and the separation of the application into several modules providing different functionality. These modules contain logic as well as UIs shown inside the application’s main UI. Engineering systems like the fdtCONTAINER application from M&M Software GmbH are typical examples. Figure 1 shows the fdtCONTAINER application screen.