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<head>
<title>UK TeX FAQ -- question label errparnum</title>
</head><body>
<h3>Illegal parameter number in definition</h3>
<p/>The error message means what it says.  In the simple case, you&rsquo;ve
attempted a definition like:
<blockquote>
<pre>
\newcommand{\abc}{joy, oh #1!}
</pre>
</blockquote><p>
or (using TeX primitive definitions):
<blockquote>
<pre>
\def\abc{joy, oh #1!}
</pre>
</blockquote><p>
In either of the above, the definition uses an argument, but the
programmer did not tell (La)TeX, in advance, that she was going to.
The fix is simple &mdash; <code>\</code><code>newcommand{<code>\</code><code>abc</code>}[1]</code>, in the
LaTeX case, <code>\def\abc#1</code> in the basic TeX case.
<p/>The more complicated case is exemplified by the attempted definition:
<blockquote>
<pre>
\newcommand{\abc}{joy, oh joy!%
  \newcommand{\ghi}[1]{gloom, oh #1!}%
}
</pre>
</blockquote><p>
will also produce this error, as will its TeX primitive equivalent:
<blockquote>
<pre>
\def\abc{joy, oh joy!%
  \def\ghi#1{gloom, oh #1!}%
}
</pre>
</blockquote><p>
This is because special care is needed when defining one macro within
the code of another macro.  This is explained elsewhere, separately
for <a href="FAQ-ltxhash.html">LaTeX definitions</a> and for
<a href="FAQ-hash.html">TeX primitive definitions</a>
<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=errparnum">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=errparnum</a>
</body>