Additional Buttons
Aftermarket manufacturers have long built mice with five or more buttons. Depending on the user's preferences and software environment, the extra buttons may allow forward and backward web-navigation, scrolling through a browser's history, or other functions, including mouse related functions like quick-changing the mouse's resolution/sensitivity. As with similar features in keyboards, however, not all software supports these functions. The additional buttons become especially useful in computer gaming, where quick and easy access to a wide variety of functions (such as macros and dpi changes) can give a player an advantage. Because software can map mouse-buttons to virtually any function, keystroke, application or switch, extra buttons can make working with such a mouse more efficient and easier.
In the matter of the number of buttons, Douglas Engelbart favored the view "as many as possible". The prototype that popularized the idea of three buttons as standard had that number only because "we could not find anywhere to fit any more switches".
On systems with three-button mice, pressing the center button (a middle click) typically opens a system-wide noncontextual menu. In the X Window System, middle-clicking by default pastes the contents of the primary buffer at the pointer's position. Many users of two-button mice emulate a three-button mouse by clicking both the right and left buttons simultaneously.
Common mice have a wheel with a detent ("bumpy" feel) to keep it from drifting accidentally; this wheel also has an optical encoder like those for the ball; it's typically used to scroll a tall window vertically. However, many such scroll wheels are mounted in a little internal spring-loaded frame so that simply pushing down on them makes them work as a third button.
Read more about this topic: Mouse Button
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