Scroll wheel
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A scroll wheel is a hard plastic or rubbery disc 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 also be used as a third mouse button by pressing on it. Some newer mice can be used to scroll horizontally as well as vertically, using either a tilting scroll-wheel (introduced by Microsoft) or a scroll ball (found on Apple's Mighty Mouse). When using Microsoft Windows, holding down the control key and using the scroll wheel causes the text size to increase or decrease, or an image in an image-editing program to zoom in or out if such a feature is available in the editing program.
The scroll wheel first gained popularity in the late 1990s, and is notable in that it was one of the first additions to the basic two-button mouse design used for PCs that became a de-facto standard. It is also one of the first hardware elements (aside from high-speed modems) designed directly in response to the proliferation of the World Wide Web (where efficient mouse-only scrolling is most useful).
Scroll wheels are quite prevalent on modern computer mice. To many users, they 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 as portable Digital audio players, PDAs or BlackBerry devices.
Recently scroll wheels have started appearing on keyboards as well, particularly on Logitech and Microsoft models. It is usually located to the left of the caps lock key.
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