A control object that belongs to a window in a graphical user interface (GUI). It normally consists of a long thin rectangle with a square marked off at each end containing an outward-pointing arrow; within the rectangle is marked a small box that can be moved along the rectangle by clicking on the arrows or inside the rectangle, or by dragging the small box. A numerical value corresponding to the box's position within the rectangle is available to the program and can be used in various ways. If it is used to move text up and down behind the window then the slider bar is known as a scroll bar, and the interior box is the scroll box. The appearance and mode of operation of the slider bar varies slightly with each GUI.