D.2 Key Binding Conventions

• Many special major modes, like Dired, Info, Compilation, and Occur, are designed to handle read-only text that contains hyper-links. Such a major mode should redefine mouse-2 and <RET> to follow the links. It should also set up a follow-link condition, so that the link obeys mouse-1-click-follows-link. See Clickable Text. See Buttons, for an easy method of implementing such clickable links.
• Don't define C-c letter as a key in Lisp programs. Sequences consisting of C-c and a letter (either upper or lower case) are reserved for users; they are the only sequences reserved for users, so do not block them.

  Changing all the Emacs major modes to respect this convention was a lot of work; abandoning this convention would make that work go to waste, and inconvenience users. Please comply with it.

• Function keys <F5> through <F9> without modifier keys are also reserved for users to define.
• Sequences consisting of C-c followed by a control character or a digit are reserved for major modes.
• Sequences consisting of C-c followed by {, }, <, >, : or ; are also reserved for major modes.
• Sequences consisting of C-c followed by any other ASCII punctuation or symbol character are allocated for minor modes. Using them in a major mode is not absolutely prohibited, but if you do that, the major mode binding may be shadowed from time to time by minor modes.
• Don't bind C-h following any prefix character (including C-c). If you don't bind C-h, it is automatically available as a help character for listing the subcommands of the prefix character.
• Don't bind a key sequence ending in <ESC> except following another <ESC>. (That is, it is OK to bind a sequence ending in <ESC> <ESC>.)

  The reason for this rule is that a non-prefix binding for <ESC> in any context prevents recognition of escape sequences as function keys in that context.

• Similarly, don't bind a key sequence ending in C-g, since that is commonly used to cancel a key sequence.
• Anything that acts like a temporary mode or state that the user can enter and leave should define <ESC> <ESC> or <ESC> <ESC> <ESC> as a way to escape.

  For a state that accepts ordinary Emacs commands, or more generally any kind of state in which <ESC> followed by a function key or arrow key is potentially meaningful, then you must not define <ESC> <ESC>, since that would preclude recognizing an escape sequence after <ESC>. In these states, you should define <ESC> <ESC> <ESC> as the way to escape. Otherwise, define <ESC> <ESC> instead.