title: How to interpret the syntax of \newcommand in latex in comparison with the command it defines
body: <p>I am new to Latex, and the question is just for curiosity. I wonder why the <code>\newcommand</code> command in latex has a different syntax than other commands?</p>
  <p>The syntax of <code>\newcommand</code> is</p>
  <pre><code>\newcommand{&lt;arg&gt;}[&lt;options&gt;][&lt;options&gt;]{&lt;arg&gt;}
  </code></pre>
  <p>while the syntax for user-defined commands (or macros?) are like</p>
  <pre><code>\&lt;command&gt;[&lt;options&gt;]{&lt;arg&gt;}{&lt;arg&gt;}
  </code></pre>
  <p>I did not find any method for a user to define a command whose use model is something like: (i.e. [] appears after {})</p>
  <pre><code>\&lt;command&gt;{&lt;args&gt;}[&lt;options&gt;]{&lt;args&gt;}
  </code></pre>
  <p>So how do we differentiate the <code>\newcommand</code> command with other user-defined commands? Call them preserved command? Some commands whose syntax not definable by the user?</p>