Bootstrap's side project [RFS](https://github.com/twbs/rfs/tree/{{< param "rfs_version" >}}) is a unit resizing engine which was initially developed to resize font sizes (hence its abbreviation for Responsive Font Sizes). Nowadays RFS is capable of rescaling most CSS properties with unit values like `margin`, `padding`, `border-radius`, or even `box-shadow`.
The mechanism automatically calculates the appropriate values based on the dimensions of the browser viewport. It will be compiled into `calc()` functions with a mix of `rem` and viewport units to enable the responsive scaling behavior.
The mixins are included in Bootstrap and are available once you include Bootstrap's `scss`. RFS can also be [installed standalone](https://github.com/twbs/rfs/tree/{{< param "rfs_version" >}}#installation) if needed.
The `rfs()` mixin has shorthands for `font-size`, `margin`, `margin-top`, `margin-right`, `margin-bottom`, `margin-left`, `padding`, `padding-top`, `padding-right`, `padding-bottom`, and `padding-left`. See the example below for source Sass and compiled CSS.
In this example, we use one of Bootstrap's built-in [responsive breakpoint mixins]({{< docsref "/layout/breakpoints" >}}) to only apply styling below the `lg` breakpoint.
RFS is a separate project under the Bootstrap organization. More about RFS and its configuration can be found on its [GitHub repository](https://github.com/twbs/rfs/tree/{{< param "rfs_version" >}}).