title: What is the exact difference between NULL and NIL in Common Lisp?
body: <p>As far as I understood, NIL is a symbol for many things: empty lists, or the boolean false. So far so good, but why there is sometimes NULL showing up in the output?</p>
  
  <pre><code>clisp&gt; (type-of NIL)
  NULL
  clisp&gt; (type-of t)
  BOOLEAN
  clisp&gt; (type-of (not t))
  NULL
  clisp&gt; NIL
  NIL
  clisp&gt; (eq NULL NIL)
  ERROR..
  </code></pre>
  
  <p>So NULL is not a defined symbol as <code>"BASIC-STRING-6"</code> is not one either. But I am a little confused by the term NULL as a boolean should stay a boolean nonetheless if it is negated or not.</p>