When you need to extend the user interface, the best place to begin is with the already defined visual and behavioral language. Look carefully at the elements that are defined in this book. Visual cues, like the drop shadow and the arrow on a pop-up menu, are triggers for people. These cues help people recognize elements that they can use.
People also learn to associate certain behaviors with specific elements.
For example, people recognize push buttons by their rounded rectangle shape. They look for a label that identifies the action the button causes. This particular appearance distinguishes a push button from other types of elements. When people click a button, they expect the button to be highlighted to indicate that the action takes effect. People may also expect that clicking a button has additional behaviors related to it, including dismissing a dialog box or changing the content area of the active document.