Using Bootstrap's visibility classes, I've created a few sample grids below for you to examine in your browser. This first grid is using all 4 grid sizes combined in a single row.
lg indicates that the large grid displaying. The grid stacks horizontally < 1200px.
md indicates that the medium grid displaying. The grid stacks horizontally < 992px.
sm indicates that the small grid displaying. The grid stacks horizontally < 768px.
xs indicates that the extra small grid displaying. This grid is always horizontal.
.col-xs-5
.col-xs-6
.col-xs-1
.col-sm-5
.col-sm-1
.col-sm-6
.col-md-1
.col-md-5
.col-md-6
.col-lg-4
.col-lg-4
.col-lg-4