Thus, to enter M-a , you could type ESC a . ESC is allowed on terminals with META keys, too, in case you have formed a habit of using it. X Windows provides several other modifier keys that can be applied to any input character. The standard key bindings of Emacs do not include any characters with these modifiers. But you can assign them meanings of your own by customizing Emacs.
Mouse buttons are also outside the gamut of characters.
Input characters and non-character inputs are collectively called input events . If you are not doing Lisp programming, but simply want to redefine the meaning of some characters or non-character events, see section Customization . These terminals use a sequence of characters to represent each function key. But that is invisible to the Emacs user, because the keyboard input routines recognize these special sequences and convert them to function key events before any other part of Emacs gets to see them. But Emacs also has commands that take two or more events to invoke.