CSS >> Bootstrap >> Grid system
<div class="row">
<div class="col-xs-12"> <!--numbers in .col-*-* should always add up to 12 for each row-->
<div class="col-xs-12 col-sm-10 col-sm-offset-1 col-md-8 col-md-offset-2"> <!--This div is centered xs12m, sm10 + offset1 * 2 = 12, md8 + offset2 * 2 = 12-->
</div>
</div>
</div>