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. 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 visible in the client area. The position of the scroll box changes whenever the user scrolls a data object to display a different portion of it. In version 4.0 or later, Windows also adjusts the size of a scroll bar's scroll box so that it indicates what portion of the entire data object is currently visible in the window. If most of the object is visible, the scroll box occupies most of the scroll bar's shaft. Similarly, if only a small portion of the object is visible, the scroll box occupies a small part of the shaft.
The user scrolls the content of a window by clicking one of the arrow buttons, by clicking the area in the shaded shaft, or by dragging the scroll box. When the user clicks an arrow button, the application scrolls the content by one unit (typically a single line or column). When the user clicks the shaded areas, the application scrolls the content by one window. The amount of scrolling that occurs when the user drags the scroll box depends on the distance the user drags the scroll box and on the scrolling range of the scroll bar. For more information about the scrolling range, see Scroll Box Position and Scrolling Range.