If dwFlags contains MOUSEEVENTF_WHEEL, then mouseData specifies the amount of wheel movement. A positive value indicates that the wheel was rotated forward, away from the user; a negative value indicates that the wheel was rotated backward, toward the user. One wheel click is defined as WHEEL_DELTA, which is 120. Windows Vista: If dwFlags contains MOUSEEVENTF_HWHEEL, then dwData specifies the amount of wheel movement. A positive value indicates that the wheel was rotated to the right; a negative value indicates that the wheel was rotated to the left. One wheel click is defined as WHEEL_DELTA, which is 120. Windows 2000/XP: IfdwFlags does not contain MOUSEEVENTF_WHEEL, MOUSEEVENTF_XDOWN, or MOUSEEVENTF_XUP, then mouseData should be zero. If dwFlags contains MOUSEEVENTF_XDOWN or MOUSEEVENTF_XUP, then mouseData specifies which X buttons were pressed or released. This value may be any combination of the following flags.