AltOrMetaKeys

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All current keyboards have a Control key (usually identified as Ctrl). When this key is pressed and held along with another key, it modifies the value sent by the key to the system. The first 32 characters in the ASCII character set are "non-printing" characters -- they do not normally have a visual representation. Many of these characters can be entered by pressing the Ctrl key at the same time as another key, and the keystroke will be interpreted as a command by the system. For example, a ^H key is normally defined as the backspace character, and ^C is the default command to terminate an operation run from a command prompt.

Keyboards may contain additional keys known as "meta" keys, which also alter the value sent when the meta key is pressed along with a regular key. On PC keyboards, the meta key is the Alt key. One common usage is to press the Alt key along with keys on the numeric keypad to generate "high" ASCII characters that represent foreign language letters, mathematical symbols, or members of the line drawing character set.

Programs can also recognize Alt-key combinations as commands, and what action the program takes when an Alt-key is pressed is often configurable.

In addition, some programs can be configured to specify just what the Alt key is. Emacs, for example, has a meta character used as a prefix to various extended commands. By default, the Meta character is the Esc key, but if you use a PC keyboard, you can tell Emacs to recognize Alt-key combinations instead.