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.

Use col-{breakpoint}-auto classes to size columns based on the natural width of their content.

Use .order- classes for controlling the visual order of your content. These classes are responsive, so you can set the order by breakpoint (e.g., .order-1.order-md-2). Includes support for 1 through 12 across all five grid tiers.

There are also responsive .order-first and .order-last classes that change the order of an element by applying order: -1 and order: 13 (order: $columns + 1), respectively. These classes can also be intermixed with the numbered .order-* classes as needed.

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

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 alignment utilities to vertically and horizontally align columns.

Use flexbox alignment utilities to vertically and horizontally align columns.