Create widgets with commands button, entry, etc:
button .b -text Quit -command { exit }

After widgets are created they have "widget commands", i.e.:
.b invoke
.b configure -foreground Blue

Widgets are arranged in a hierarchical structure. By nature, a widget named .parent.child is a "child widget" of the widget named .parent . There exists by default a main window, called ".", which is the ultimate parent of all other widgets. It is possible to create other toplevel windows with the "toplevel" command, but usually the default main window suffices.