Using a single set of .col-sm-* classes, can you create a basic grid system that starts out stacked and becomes horizontal at the small breakpoint (*sm).
Don’t want your columns to simply stack in some grid tiers, use a combination of different classes for each tier as needed.
Mixins are used in conjunction with the grid variables to generate CSS for individual grid columns.
You can modify the variables to your own custom values, or just use the mixins with their default values. Here a example of using the default settings to create a two-column layout with a gap between.