Push button form fields are most commonly used for implementing application execution control. Push buttons have no associated data and by default, are assigned to respond to the "Click" callback event. Push buttons can be either text-based or image-based. A text push button appears as the familiar raised gray "bar" with superimposed text. The physical dimensions of a text push button can be changed as required. An image push button appears as the familiar raised gray bar with a superimposed image which can be a bit map, an icon or Windows meta file. The physical dimensions of an image push button are controlled by the selected automatic sizing option. A push button can also be defined as being the "default push button". The default push button is the one which is considered to have been selected if the Enter key is pressed, and is displayed with a thicker border than the other push buttons.