A scroll bar has three main parts: the bar, its arrows, and a slider.
The arrows are located at each end of the scroll bar. The left scroll arrow, on the left side of a horizontal scroll bar, enables the user to scroll to the left in a document. The right scroll arrow lets the user scroll to the right.
On a vertical scroll bar, the upper scroll arrow enables the user to scroll upward in the document; the lower scroll arrow, downward. The slider, which lies between the two scroll arrows, reflects the current value of the scroll bar. Scroll bars monitor the slider and send notification messages to the owner window when the slider position changes as a result of mouse or keyboard input.
Although, typically, scroll bars are used in frame windows, an application can use stand-alone scroll bars of any size or shape, at any position, in a window of almost any class. Scroll bars can be used as parts of other control windows; for example, a list box uses a scroll bar to enable the user to view items when the list box is too small to display all the items.