30.4.8 Named ASCII Control Characters

<TAB>, <RET>, <BS>, <LFD>, <ESC> and <DEL> started out as names for certain ASCII control characters, used so often that they have special keys of their own. Later, users found it convenient to distinguish in Emacs between these keys and the “same” control characters typed with the <CTRL> key.

Emacs distinguishes these two kinds of input, when the keyboard reports these keys to Emacs. It treats the “special” keys as function keys named tab, return, backspace, linefeed, escape, and delete. These function keys translate automatically into the corresponding ASCII characters if they have no bindings of their own. As a result, neither users nor Lisp programs need to pay attention to the distinction unless they care to.

If you do not want to distinguish between (for example) <TAB> and C-i, make just one binding, for the ASCII character <TAB> (octal code 011). If you do want to distinguish, make one binding for this ASCII character, and another for the “function key” tab.

With an ordinary ASCII terminal, there is no way to distinguish between <TAB> and C-i (and likewise for other such pairs), because the terminal sends the same character in both cases.