title: Bootstrap 3: is col-xs-12 optional?
body: <p>We have</p>
  
  <pre><code>&lt;div class="col-md-6"&gt;left&lt;/div&gt;
  &lt;div class="col-md-6"&gt;right&lt;/div&gt;
  </code></pre>
  
  <p>For smaller screens than <code>md</code>, we'd like to have it in two lines. Looks like it is displayed as expected. Perfect.</p>
  
  <p>The same result is when we add <code>col-xs-12</code> so it is </p>
  
  <pre><code>&lt;div class="col-xs-12 col-md-6"&gt;left&lt;/div&gt;
  &lt;div class="col-xs-12 col-md-6"&gt;right&lt;/div&gt;
  </code></pre>
  
  <p>That's  one line for large screens and two lines for small screens.</p>
  
  <p><strong>The question is:</strong> </p>
  
  <p>As result, is <code>col-xs-12</code> class optional in this case or it is recommended?</p>