基本的なグリッドシステム

Window Width :

col-sm-*

576px 未満(xs):1カラム

col-sm-8
col-sm-4
col-sm
col-sm
col-sm
<div class="container">
  <div class="row">
    <div class="col-sm-8">col-sm-8</div>
    <div class="col-sm-4">col-sm-4</div>
  </div>
  <div class="row">
    <div class="col-sm">col-sm</div>
    <div class="col-sm">col-sm</div>
    <div class="col-sm">col-sm</div>
  </div>
</div> 

col-md-*

768px 未満(xs と sm):1カラム

col-md-8
col-md-4
col-md
col-md
col-md
<div class="container">
  <div class="row">
    <div class="col-md-8">col-md-8</div>
    <div class="col-md-4">col-md-4</div>
  </div>
  <div class="row">
    <div class="col-md">col-md</div>
    <div class="col-md">col-md</div>
    <div class="col-md">col-md</div>
  </div>
</div>

col-lg-*

992px 未満(xs, sm, md):1カラム

col-lg-8
col-lg-4
col-lg
col-lg
col-lg
<div class="container">
  <div class="row">
    <div class="col-lg-8">col-lg-8</div>
    <div class="col-lg-4">col-lg-4</div>
  </div>
  <div class="row">
    <div class="col-lg">col-lg</div>
    <div class="col-lg">col-lg</div>
    <div class="col-lg">col-lg</div>
  </div>
</div>