A scroll wheel (or mouse wheel) is a hard plastic or rubbery disc (the "wheel") on a computer mouse that is perpendicular to the mouse surface. It is normally located between the left and right mouse buttons.
It is used, as the name suggests, for scrolling. It can often also be used as a third mouse button by pressing on it. The wheel is often – but not always – engineered with detents to turn in discrete steps, rather than continuously as an analog axis, to allow the operator to more easily intuit how far they are scrolling.
Some modern mice can scroll horizontally as well as vertically, using a tilting scroll-wheel (introduced by Microsoft), scroll ball (found, for example, on Apple's Mighty Mouse), pointing stick, touchpad (as on Apple's Magic Mouse), optical sensor, or a second scroll wheel. Logitech mice with 'Free Spinning' scroll wheels use a 14 gram scroll wheel with high rotational inertia like a flywheel that can be used to quickly scroll through long pages and lists. A peculiar early example was a mouse by Saitek which had a joystick-style hatswitch on it.
Scroll wheels are prevalent on modern computer mice and have become an integral part of the hardware interface. However, non-wheeled mice are still available.
Scroll wheels can also be found on such handheld devices such as PDAs, portable digital audio players such as the original Apple iPod, or mobiles phones such as early Sony models and BlackBerry devices.
In 1985 NTT, Japan and ETH Zürich, Switzerland (Kunio Ōno, Ken'ichi Fukaya and Jürg Nievergelt) jointly invented and developed the first scrolling mouse. Their Mighty Mouse had the scroll wheel accessible on the side. In 1987, an early prototype of the top mounted scrolling wheel mouse was developed by Jack McCauley for gaming applications. Between 1989 and 1993 Daniel S. Venolia of Apple developed another mouse prototype with a thumb-wheel accessible on the side.
The scroll wheel was popularized by the Microsoft IntelliMouse in 1996 along with support for the mouse wheel in Microsoft Office 97. It was based on ideas developed by Eric Michelman since 1993 with input from Chris Graham.
The introduction of the scroll wheel is notably one of the first additions to the basic two-button mouse design used for PCs that became a de facto standard. Its popularity increased with the proliferation of the World Wide Web, where efficient mouse-only scrolling is most useful, since it allows the mouse pointer to remain close to the page's hyperlinks while scrolling rather than moving to a scroll bar.
In the 21st century, scroll wheels started appearing on keyboards as well, particularly on Logitech and Microsoft models. It is usually located to the left of the caps lock key. The implementation of scroll wheels on laptop computers has generally faded, while touchpads are often programmed with a pointing device gesture to substitute for them; such as allowing the edges to scroll the page (rather than to move the pointer), partly making up for the lack of a scroll wheel; touchpads with multitouch capability usually achieve scroll capability by touching and dragging two fingers on the touchpad at the same time; many Linux distributions offer a third method of scrolling using the touchpad, where the user will first activate scroll-mode by pressing in a corner of the pad, and then dragging in a circle around the center of the pad, letting go of the touchpad will switch back to the default mouse-mode.
In many applications (e.g., web browsers), holding down the control key while rolling the scroll wheel causes the text size to increase or decrease, or an image in an image-editing or map-viewing program to zoom in or out, if such a feature is available. Also clicking the link with a scroll wheel button makes the link open in a new tab, and clicking on the opened tab closes it.
In first-person shooter (FPS) games, the scroll wheel is usually set to change weapons. Because of this some gaming mice, such as Logitech G500, implement a locking feature that lets the mouse switch between scrolling in steps (needed for gaming) and continuous scrolling (good for web browsing). The scroll wheel is also sometimes used to control the zoom of scoped weapons and binoculars. FPS games with an emphasis on realism may choose to use the wheel for changing between stances or adjusting the range settings on iron sights. Since the introduction of tilting scroll-wheels, many FPS games use the left-right motion of the scroll wheel to cause the player to lean left and right. A less common use is to use the mouse wheel to adjust the player's speed of movement.
Real time strategy games often use the mouse wheel to change the altitude of the camera; many also allow the view to either tilt or pan with mouse movement when the scroll wheel button is held down.
CAD applications such as AutoDesk's AutoCAD use the mouse wheel to navigate the space in which the user is drawing. This has also become a de facto standard in 3D applications such as Trimble's SketchUp where the wheelscroll is zoom, and wheelclick with a mouse drag is orbit.