The horizontal and vertical scroll bars, bars on the right and lower sides of a window, let a user scroll the contents of the client area. Windows sends scroll requests to a window as WM_HSCROLL and WM_VSCROLL messages. If the window permits scrolling, the window function must process these messages.
A window can have one or both scroll bars. To create a window with a scroll bar, the application must specify the WS_HSCROLL or WS_VSCROLL window style when the window is created.