The horizontal and vertical scroll bars are bars on the lower and right sides of a window, respectively, making it possible for a user to 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 procedure 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. An application can use the ShowScrollBar function to show or hide a scroll bar of a window with the WS_HSCROLL or WS_VSCROLL style.