    <div class="col-md-4">.col-md-4</div>
      </div>

      <hr>

      <h3>Mixed: mobile and desktop</h3>
      <p>The Bootstrap v4 grid system has five tiers of classes: xs (extra small), sm (small), md (medium), lg (large), and xl (extra large). You can use nearly any combination of these classes to create more dynamic and flexible layouts.</p>
      <p>Each tier of classes scales up, meaning if you plan on setting the same widths for xs and sm, you only need to specify xs.</p>
      <div class="row">
