Bootstrap 3 and .col-xs-* — Do you not need rows of 12 units?

Question

I\'m a little confused by the Bootstrap 3 documentation and thus usage of the .col-xs-* classes.

The docs for Grid Options say that all of the grid systems

Answer 1

The amount of rows a column occupies is the last number of the class.

So for example, these following classes:

.col-xs-12 .col-md-8  
.col-xs-6 .col-md-4

will result in a single row on the md-width displays but one and a half row on xs-width displays.
This simply means that on small displays those elements won't display side-by-side, but instead on top of each other.