Kinds of User Input
GNU Emacs uses an extension of the ASCII character set for keyboard input; it also accepts non-character input events including function keys and mouse button actions.
ASCII consists of 128 character codes. Some of these codes are assigned graphic symbols such as a and =; the rest are control characters, such as Control-a (usually written C-a for short). C-a gets its name from the fact that you type it by holding down the <CTRL> key while pressing a.
Some ASCII control characters have special names, and most terminals have special keys you can type them with: for example, <RET>, <TAB>, <DEL> and <ESC>. The space character is usually referred to below as <SPC>, even though strictly speaking it is a graphic character whose graphic happens to be blank. Some keyboards have a key labeled "linefeed" which is an alias for C-j.
Emacs extends the ASCII character set with thousands more printing characters (see International), additional control characters, and a few more modifiers that can be combined with any character.
On ASCII terminals, there are only 32 possible control characters. These are the control variants of letters and @[]\^_. In addition, the shift key is meaningless with control characters: C-a and C-A are the same character, and Emacs cannot distinguish them.
But the Emacs character set has room for control variants of all printing characters, and for distinguishing between C-a and C-A. The X Window System makes it possible to enter all these characters. For example, C-- (that's Control-Minus) and C-5 are meaningful Emacs commands under X.
Another Emacs character-set extension is additional modifier bits. Only one modifier bit is commonly used; it is called Meta. Every character has a Meta variant; examples include Meta-a (normally written M-a, for short), M-A (not the same character as M-a, but those two characters normally have the same meaning in Emacs), M-<RET>, and M-C-a. For reasons of tradition, we usually write C-M-a rather than M-C-a; logically speaking, the order in which the modifier keys <CTRL> and <META> are mentioned does not matter.
Some terminals have a <META> key, and allow you to type Meta characters by holding this key down. Thus, Meta-a is typed by holding down <META> and pressing a. The <META> key works much like the <SHIFT> key. Such a key is not always labeled <META>, however, as this function is often a special option for a key with some other primary purpose. Sometimes it is labeled <ALT> or <EDIT>; on a Sun keyboard, it may have a diamond on it.
If there is no <META> key, you can still type Meta characters using two-character sequences starting with <ESC>. Thus, you can enter M-a by typing <ESC> a. You can enter C-M-a by typing <ESC> C-a. <ESC> is allowed on terminals with <META> keys, too, in case you have formed a habit of using it.
The X Window System provides several other modifier keys that can be applied to any input character. These are called <SUPER>, <HYPER> and <ALT>. We
