With video games that use side-scrolling, often the screen will scroll forward following the speed and direction of the player character, and can also scroll backwards to previously visited parts of a stage. In other games or stages the screen will follow the player character but only scroll forward, not backwards, so once something has passed off the back of the screen it can no longer be visited. Some games have stages where the screen scrolls forward by itself at a steady rate, and the player must keep up with the screen, attempting to avoid obstacles and collect things before they pass off screen. The screen in shoot 'em ups such as R-type often side-scrolls by itself in such a way. The "Mario" series has used all of three of these different ways of side-scrolling.