The Alt key on a computer keyboard is used to change (alternate) the function of other pressed keys. Thus, the Alt key is a modifier key, used in a similar fashion to the Shift key. For example, simply pressing "A" will type the letter a, but if you hold down either Alt key while pressing A, the computer will perform an "Alt-A" function, which varies from program to program. In non-US keyboard layouts, rather than a second Alt key, there is an almost identical 'Alt Gr' key to the right of the space bar. The key is located immediately to either side of the Space bar.
The Alt key has come to replace the Meta key of the old MIT keyboards. In their original function, both Alt and Meta would set the high bit of the signal generated by the key to 1 (for example, A generates 01000001 while Alt-A generates 11000001). However, in modern software, due to the requirement of the high bit for internationalisation, Alt no longer works in such a way.
The Alt key is well known as part of the Control-Alt-Delete key combination, which in some operating systems brings up the task manager for aborting an unresponsive application (or restarting the computer) or simply viewing what applications or processes are running.
Other well-known combinations which the Alt key is part of include Alt-F4, to close a window, and Alt-Tab, to switch between windows. Additionally, in many traditional GUI environments, including Microsoft Windows, Alt is used to access pull-down menus.
Some keyboard layouts treat both Alt keys on the keyboard as the same key, while others do not.
In some software, holding down the Alt key while typing in numbers (often referred to as Alt codes) on the numeric keypad allows the user to type special characters not normally available on the keyboard. For example, holding down Alt while typing 0169 on the numeric keypad will result in ©. These extended keyboard characters are useful for persons using foreign languages, mathematics, currency symbols, business use, etc.