Nearly all event messages apply to a single window’s responder chain—the window in which the associated user event occurred. The default responder chain for event messages begins with the view that the NSWindow object first delivers the message to. The default responder chain for a key event message begins with the first responder in a window; the default responder chain for a mouse or tablet event begins with the view on which the user event occurred. From there the event, if not handled, proceeds up the view hierarchy to the NSWindow object representing the window itself. The first responder is typically the “selected” view object within the window, and its next responder is its containing view (also called its superview), and so on up to the NSWindow object. If an NSWindowController object is managing the window, it becomes the final next responder. You can insert other responders between NSView objects and even above the NSWindow object near the top of the chain. These inserted responders receive both event and action messages. If no object is found to handle the event, the last responder in the chain invokes noResponderFor:, which for a key-down event simply beeps. Event-handling objects (subclasses of NSWindow and NSView) can override this method to perform additional steps as needed.