title: why class="btn btn-default" instead of only class="btn-default"
body: <p>In bootstrap one thing that is kicking in my ass since many days is that I don't understand the concept behind the tag when including bootstrap classes in HTML.</p>
  
  <p>Like -</p>
  
  <pre><code>&lt;button class="btn btn-primary"&gt; press me &lt;/button&gt;
  </code></pre>
  
  <p>why there is two? btn and btn-primary. why we don't only include only btn-primary as we want a primary type btn only. What does it mean and what is the purpose of it ?</p>
  
  <p>there are also many examples like this - </p>
  
  <pre><code> &lt;ul class="nav navbar-nav navbar-right"&gt;
  </code></pre>