In Bootstrap it's possible to use grid columns inside other columns. This is helpful when creating advanced responsive layouts that utilize multiple grid tiers.
We can have as many number of columns as possible in the above mentioned way.
<div class="row"> <div class="col-sm-9"> Level 1: .col-sm-9 <div class="row"> <div class="col-xs-8 col-sm-6"> Level 2: .col-xs-8 .col-sm-6 </div> <div class="col-xs-4 col-sm-6"> Level 2: .col-xs-4 .col-sm-6 </div> </div> </div> </div>
The example is taken from [http://getbootstrap.com/css/#grid-nesting]
as the documentation of official website suggests
To nest your content with the default grid, add a new .row and set of .col-sm-* columns within an existing .col-sm-* column. Nested rows should include a set of columns that add up to 12 or fewer (it is not required that you use all 12 available columns).