.col-sm-12 .col-md-6
.col-sm-12 .col-md-6
.col-sm-12 .col-md-12

<div class="row">
    <div class="col-sm-12 col-md-6">
        <span>.col-sm-12 .col-md-6</span>
    </div>
    <div class="col-sm-12 col-md-6">
        <span>.col-sm-12 .col-md-6</span>
    </div>
    <div class="col-sm-12 col-md-12">
        <span>.col-sm-12 .col-md-12</span>
    </div>
</div>