The scroll wheel is one of the few improvements to the basic computer mouse that has achieved near ubiquity, at least for IBM clones. It is a small, wide dial mounted vertically between the left and right mouse buttons, a disk with an axis running between the two buttons. Also, in most implementations the wheel can be pressed into the body of the mouse, providing a difficult-to-use "clickability".

The scroll wheel appeared in the mid-90s and quickly gained popularity, appearing on several Logitech models as well as the Microsoft IntelliMouse.

Most often it is used for scrolling the current window, where it saves having to bring the pointer to the right side of the window, locating the page marker within the scroll bar, and "left-dragging". It is a pretty significant ergonomic improvement in that regard. Under Windows, there is also an odd "scroll-drag" (clicking or holding down the scroll wheel and moving the mouse) which starts a steady variable speed scroll of the current window.

Helpful Hints: you may find it useful to go into the options and set the mouse to scroll one page at a time rather than the default three lines. Also, under Internet Explorer ctrl-scroll will raise and lower non-CSS fonts through the five available size settings.