The user-defined signal handlers are called ~callbacks~. The first parameter the callback receives is the widget that emitted the signal. The rest are signal-dependent. For example, the callback for %switch_page% signal of *GtkNotebook* class would look like this: ]]> where %$page% is the object representing the page that was switched to, and %$page_num% is its page number. The low-level event callbacks receive an additional object describing the event. It has properties for event type, button number if it was a mouse click, key code if it was a key press, etc. It is possible to connect more than one callback to the same signal. They will be run in the order they are connected. It is also possible to simulate emission of a signal by the widget, for some custom purposes.