Alt key

The Alt key on a computer keyboard is used to change 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 holding down the Alt key while pressing A will cause the computer to perform an function, which varies from program to program. The international standard ISO/IEC 9995-2 calls it Alternate key. The key is located on either side of the space bar, but in non-US PC keyboard layouts, rather than a second Alt key, there is an 'Alt Gr' key to the right of the space bar. Both placements are in accordance with ISO/IEC 9995-2. With some keyboard mappings, the right Alt key can be reconfigured to function as an AltGr key although not engraved as such.
The standardized keyboard symbol for the Alt key, is given in ISO/IEC 9995-7 as symbol 25, and in ISO 7000 "Graphical symbols for use on equipment" as symbol ISO-7000-2105. This symbol is encoded in Unicode as. Macintosh keyboards equate the Alt key with the key, which has its own symbol, similar to that of the ISO standard.
The Alt key should not be confused with the Altmode key on some Teletype and ASCII terminals, which is a synonym for the ASCII escape character.

Evolution

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. However, in modern software, due to the requirement for all bits to be used for post-ASCII 8-bit character sets, Alt no longer works in such a way.
Since the 1990s, Alt has been printed on the Option key on most Mac keyboards. As of 2017, the newest Apple keyboards do not contain the Alt label. Alt is used in non-Mac software, such as non-macOS Unix and Windows programs, but in macOS it is always referred to as the Option key. The Option key's behaviour in macOS differs slightly from that of the Windows Alt key.

Alt key combinations

The key is well known as part of various standard combinations:

may restart the computer ; in some operating systems it brings up a task manager - see Control-Alt-Delete.
is treated as by Windows, although some software do not accept this mapping.
brings up the Force Quit window in Apple's macOS, see Option key.
usually causes the X display server to shut down or to restart, see Control-Alt-Backspace.
closes the current window on most windowing systems.
switches between language layouts.
switches between the currently opened windows on most windowing systems, often referred to as alt-tabbing.
allows for a new line when alone would perform another command. For example, in a word processor, this creates a line break rather than a paragraph break.
Holding allows the selection of text within a hyperlink as if it were solid text. On Linux, this is done using.
When only holding Alt while dragging with the mouse on Linux, the window is moved.

Alt key for special characters

Windows

In Microsoft Windows, holding down the Alt key while typing in numbers on the numeric keypad allows the user to type special characters not normally available on the keyboard. For example, holding down Alt while typing 0225 on the numeric keypad will result in, the character at 225 in the code page 1252. These extended keyboard characters are useful for persons using foreign languages, mathematics, currency symbols, business use, etc. Some computers work the same if the Num Lock key is on.

macOS

On a Macintosh, the Alt key is called the Option key. It is not used to enter numeric character codes. Instead, keyboard letters and numbers are used. The diagram below shows the special characters a US Mac keyboard will produce when the Option key is pressed.
The highlighted orange keys show the accents available from the combination of the key and the keyboard characters and . The accent then can be applied to associated letters both lower and uppercase. The additional characters a Mac will produce are a combination of both the key and the key pressed down together. With this combination pressed the keyboard will now produce a different set or an uppercase version of the previous set in some cases.

Alt key pressed alone

When a user presses the key by itself on Microsoft Windows, that moves keyboard focus to the menu bar of the application having keyboard focus, and the key is not delivered to the application. In that state, another press of the key will be delivered to the application.
On other platforms, for example Ubuntu, the key pressed by itself may invoke another application and not be delivered to the application having keyboard focus. In Ubuntu 16.04, pressing the key opens the HUD, which is an input field that lets the user perform menu commands such as opening a file, or creating a new file.