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From: Eli Zaretskii <eliz@elta.co.il>
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Subject: Re: What does the coding system nil mean?
Date: 25 Jan 2004 08:10:58 +0200
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In-reply-to: <m365f1571p.fsf@defun.localdomain> (message from Jesper Harder on
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> Newsgroups: gnu.emacs.help
> From: Jesper Harder <harder@myrealbox.com>
> Date: Sat, 24 Jan 2004 23:13:38 +0100
> 
> So, can anyone explain what a nil coding system means?

It means, quite naturally, that no code-conversion should take place.
That is, the original text or string are left unaltered.

Try it:

  M-: (encode-coding-string "SOME STRING" nil) 
      => "SOME STRING"

(include non-ASCII characters in the string to convince yourself they
are not encoded).

I agree that this should be in the docs somewhere, e.g. in the docs of
the en/decoding primitives such as encode-coding-region and
decode-coding-string.  So a documentation bug report is probably in
order.