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 keyboard input character. Not all X terminals actually provide keys for these modifier flags, and the standard key bindings of Emacs do not include such characters. But you can assign them meanings of your own by customizing Emacs.
Mouse buttons are also outside the gamut of characters.
These inputs do not have numeric character codes. Input characters and non-character inputs are collectively called input events . 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 converts them to names before any other part of Emacs gets to see them.