To create a new horizontal box, we use a call to GtkHBox(), and for vertical boxes, GtkVBox(). The pack_start() and pack_end() methods are used to place objects inside of these containers. The pack_start() method will start at the top and work its way down in a vbox, and pack left to right in an hbox. pack_end() will do the opposite, packing from bottom to top in a vbox, and right to left in an hbox. Using these methods allows us to right justify or left justify our widgets and may be mixed in any way to achieve the desired effect. We will use pack_start() in most of our examples. An object may be another container or a widget. In fact, many widgets are actually containers themselves, including the button, but we usually only use a label inside a button.
By using these calls, GTK knows where you want to place your widgets so it can do automatic resizing and other nifty things. There are also a number of options as to how your widgets should be packed. As you can imagine, this method gives us a quite a bit of flexibility when placing and creating widgets.