col-md bootstrap 4

.row{
    display: flex;
    flex-wrap: wrap;
	margin-right: -15px;
    margin-left: -15px;
}

.col-12{
	flex: 0 0 100%;
	max-width: 100%;
}

<div class="row">
    <div class="col-xs-12 col-md-8">.col-xs-12 col-md-8</div>
    <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
</div>
<div class="row">
  <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
  <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
  <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
</div>
<div class="row">
  <div class="col-xs-6">.col-xs-6</div>
  <div class="col-xs-6">.col-xs-6</div>
</div>