A scroll bar consists of a shaded shaft with an arrow button at each end and a scroll box (sometimes called a thumb) between the arrow buttons. The following illustration shows the parts of a scroll bar:
A scroll bar represents the overall length or width of a data object in a window's client area; the scroll box represents the portion of the object that is currently visible in the client area. When the data object is scrolled so that a different portion of the object is visible, the position of the scroll box changes.
The user scrolls the contents of a window by clicking a scroll bar's arrow buttons, clicking in the shaded shaft, or dragging the scroll box. When the user clicks an arrow button, your application should scroll the contents by one unit (typically a single line or column). When the user clicks the shaded areas, your application should scroll the contents by one windowful. The amount of scrolling that should occur when the user drags the scroll box depends on the distance the user drags the scroll box and the scroll bar's scrolling range. For more information about the scrolling range, see Section 0.1.3, “Scroll-Box Position and Scrolling Range.”