title: Is it necessary to add col-md selector in Bootstrap 3?
body: <p>I tried searching for an answer to this to no avail.  Basically what I'm wondering is if it is necessary to specify a col-md-* value in Bootstrap 3, or if you can just use, say, col-xs-12 and col-sm-* and have the col-sm-* value dictate the col size all the way to some massive display. </p>
  
  <p>This seems like a no-brainer (that having col-sm would be fine) but the documentation includes seemingly redundant col-md-* values as well, such as here (taken from "Example: Mobile, Tablet, and Desktop" under "Grid System" in CSS section:</p>
  
  <pre><code>&lt;div class="row"&gt;
    &lt;div class="col-xs-6 col-sm-4 col-md-4"&gt;.col-xs-6 .col-sm-4 .col-md-4&lt;/div&gt;
    &lt;div class="col-xs-6 col-sm-4 col-md-4"&gt;.col-xs-6 .col-sm-4 .col-md-4&lt;/div&gt;
    &lt;!-- Optional: clear the XS cols if their content doesn't match in height --&gt;
    &lt;div class="clearfix visible-xs"&gt;&lt;/div&gt;
    &lt;div class="col-xs-6 col-sm-4 col-md-4"&gt;.col-xs-6 .col-sm-4 .col-md-4&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Any clarification here would be greatly appreciated. Thanks!</p>