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## Start

After starting XF , two toplevels appear on the screen. The empty toplevel window is the workspace where the application is to be built. The second window is the main XF window (see figure ).

Typically, a Tcl/Tk application has one main dialog window (the Tk main window). From here, the other dialog windows and the functionality of the application are activated (displayed). Dialog components that do not need to be in the main dialog window can be placed in additional toplevel windows (like option settings or alert boxes). A toplevel window is almost the same as the main Tk window, but it can be destroyed, while the main Tk window cannot be destroyed. So it makes sense to put dialog elements that have to be displayed permanently in the main Tk window, and dialog elements that are used only temporarily into toplevel windows. The toplevel windows and the main Tk

window contain the widgets that form the interface of the application. Widgets can contain other widgets forming the widget tree. Higher-level widgets are used as containers to layout the leaf widgets that implement the functionality.