Answers for "The Bootstrap grid system is based on how many columns?"

bootstrap grid 2 rows

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet"/>


<div class="row">
    <div class="col-md-4">
        <div class="well">1
            <br/>
            <br/>
            <br/>
            <br/>
            <br/>
        </div>
    </div>
    <div class="col-md-8">
        <div class="row">
            <div class="col-md-6">
                <div class="well">2</div>
            </div>
            <div class="col-md-6">
                <div class="well">3</div>
            </div>
        </div>
        <div class="row">
            <div class="col-md-6">
                <div class="well">4</div>
            </div>
            <div class="col-md-6">
                <div class="well">5</div>
            </div>
        </div>
    </div>
</div>
<div class="row">
    <div class="col-md-4">
        <div class="well">6</div>
    </div>
    <div class="col-md-4">
        <div class="well">7</div>
    </div>
    <div class="col-md-4">
        <div class="well">8</div>
    </div>
</div>

bootstrap screen sizes

The Bootstrap grid system has four classes:
xs (for phones - screens less than 768px wide)
sm (for tablets - screens equal to or greater than 768px wide)
md (for small laptops - screens equal to or greater than 992px wide)
lg (for laptops and desktops - screens equal to or greater than 1200px wide)

a column inside another column bootstrap html

<div class="row">
  <div class="col-sm-9">
    Level 1: .col-sm-9
    <div class="row">
      <div class="col-8 col-sm-6">
        Level 2: .col-8 .col-sm-6
      </div>
      <div class="col-4 col-sm-6">
        Level 2: .col-4 .col-sm-6
      </div>
    </div>
  </div>
</div>