The most commonly used computer pointing device, first introduced by Douglas Engelbart in 1968. The mouse is a device used to manipulate an on-screen pointer that's normally shaped like an arrow. With the mouse in hand, the computer user can select, move, and change items on the screen.
A conventional roller-ball mouse is slid across the surface of the desk, often on a mouse mat. As the mouse moves, a ball set in a depression on the underside of the mouse rolls accordingly. The ball is also in contact with two small shafts set at right angles to each other inside the mouse. The rotating ball turns the shafts, and sensors inside the mouse measure the shafts' rotation. The distance and direction information from the sensors is then transmitted to the computer, usually through a connecting wire - the mouse's "tail". The computer then moves the mouse pointer on the screen to follow the movements of the mouse. This may be done directly by the graphics adaptor, but where it involves the processor the task should be assigned a high priority to avoid any perceptible delay.
Some mice are contoured to fit the shape of a person's right hand, and some come in left-handed versions. Other mice are symmetrical.
Included on the mouse are usually two or three buttons that the user may press, or click, to initiate various actions such as running programs or opening files. The left-most button (the primary mouse button) is operated with the index finger to select and activate objects represented on the screen. Different operating systems and graphical user interfaces have different conventions for using the other button(s). Typical operations include calling up a context-sensitive menu, modifying the selection, or pasting text. With fewer mouse buttons these require combinations of mouse and keyboard actions. Between its left and right buttons, a mouse may also have a wheel that can be used for scrolling or other special operations defined by the software. Some systems allow the mouse button assignments to be swapped round for left-handed users.
Just moving the pointer across the screen with the mouse typically does nothing (though some CAD systems respond to patterns of mouse movement with no buttons pressed). Normally, the pointer is positioned over something on the screen (an icon or a menu item), and the user then clicks a mouse button to actually affect the screen display.
The five most common "gestures" performed with the mouse are: point (to place the pointer over an on-screen item), click (to press and release a mouse button), double-click to press and release a mouse button twice in rapid succession, right-click (to press and release the right mouse button}, and drag (to hold down the mouse button while moving the mouse).
Most modern computers include a mouse as standard equipment. However, some systems, especially portable laptop and notebook models, may have a trackball, touchpad or Trackpoint on or next to the keyboard. These input devices work like the mouse, but take less space and don't need a desk.
Many other alternatives to the conventional roller-ball mouse exist. A tailless mouse, or hamster, transmits its information with infrared impulses. A foot-controlled mouse is one used on the floor underneath the desk. An optical mouse uses a light-emitting diode and photocells instead of a rolling ball to track its position. Some optical designs may require a special mouse mat marked with a grid, others, like the Microsoft IntelliMouse Explorer, work on nearly any surface.