All Presentation Manager applications have a similar structure. The application is composed of a main routine, which performs initialization and termination functions, and which contains the application's main message processing loop, and a number of window procedures that process messages for window classes created and/or used by the application. These window procedures are invoked and messages passed to them by Presentation Manager on behalf of the application, as shown in Figure "Message Flow in a Presentation Manager Application".
Window procedures may also pass messages between one another in order to communicate events. The flow of messages between the window procedures is also controlled by Presentation Manager on behalf of the application.