While Bootstrap uses ems or rems for defining most sizes, pxs are used for grid breakpoints and container widths. This is because the viewport width is in pixels and does not change with the font size.

See how aspects of the Bootstrap grid system work across multiple devices with a handy table.

Using a single set of .col-md-* grid classes, you can create a basic grid system that starts out stacked on mobile devices and tablet devices (the extra small to small range) before becoming horizontal on desktop (medium) devices. Place grid columns within any .row.

Don’t want your columns to simply stack in smaller devices? Use the extra small and medium device grid classes by adding .col-xs-* and .col-md-* to your columns. See the example below for a better idea of how it all works.

Don’t want your columns to simply stack in smaller devices? Use the extra small and medium device grid classes by adding .col-xs-* and .col-md-* to your columns. See the example below for a better idea of how it all works.

Move columns to the right using .*-auto classes. These classes increase the left margin of a column by * columns. For example, .ml-auto moves .col-md-4 over four columns.

Easily change the order of our built-in grid columns with .push-md-* and .pull-md-* modifier classes.

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).

Use flexbox utilities for controlling the visual order of your content.