One major innovation in mouse buttons was the scroll wheel: a small wheel, with its axis oriented parallel to the mousing surface, that could be rotated “up” or “down” to provide immediate one-dimensional input. Usually, this input was translated into moving up or down within the currently selected window. This is especially helpful in navigating a long document. The scroll wheel can often be pressed straight down, replacing the third (center) button. Doing so can activate auto-scrolling in the Windows operating system if the application supports it. The wheel can also be used with some software applications to zoom in and out, by holding down the Control key (Ctrl) on the keyboard and scrolling either up or down. In a WIMP-style graphical user interface (GUI), scrolling is done with the help of a scrollbar or using keyboard shortcuts, often the arrow keys.
In 2005 the Apple Mighty Mouse (codenamed ‘Houdini’) introduced another variety of built-in scrolling device. The “scroll ball” is essentially a small trackball embedded in the upper surface of the mouse, and is used like a wheel but in two dimensions, allowing both horizontal and vertical scrolling.