看了别人的代码不理解加粗部分

<div class="row">

         <div class="col-md-1 col col-sm-2"></div>

        <div class="col-md-1 col col-sm-2"></div>

         <div class="col-md-2 col col-sm-8"></div>

        <div class="col-md-2 col col-sm-3"></div>

         <div class="col-md-6 col col-sm-3"></div>

</div>
一行里应该是12。加粗的类名超了啊，解释下