css/modules/primer-support/docs/breakpoints.md

title	status	source
Breakpoints	Stable	https://github.com/primer/primer/blob/master/modules/primer-support/lib/mixins/layout.scss

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Our breakpoints are based on screen widths where our content starts to break. Since most of GitHub is currently a fixed-width with we use pixel widths to make it easy for us to match page widths for responsive and non-responsive pages. Our breakpoints may change as we move more of the product into responsive layouts.

We use abbreviations for each breakpoint to keep the class names concise. This abbreviated syntax is used consistently across responsive styles. Responsive styles allow you to change the styles properties at each breakpoint. For example, when using column widths for grid layouts, you can change specify that the width is 12 columns wide at the small breakpoint, and 6 columns wide from the large breakpoint: <div class="col-sm-12 col-lg-6">...</div>

Breakpoint	Syntax	Description
Small	sm	min-width: 544px
Medium	md	min-width: 768px
Large	lg	min-width: 1012px
Extra-large	xl	min-width: 1280px

Note: The lg breakpoint matches our current page width of 980px including left and right padding of 16px ($spacer-3). This is so that content doesn't touch the edges of the window when resized.

Responsive styles are available for margin, padding, layout, flexbox, and the grid system.

Breakpoint variables

The above values are defined as variables, and then put into a Sass map that generates the media query mixin.

// breakpoints
$width-xs: 0;
$width-sm: 544px;
$width-md: 768px;
$width-lg: 1012px;
$width-xl: 1280px;

$breakpoints: (
  // Small screen / phone
  sm: $width-sm,
  // Medium screen / tablet
  md: $width-md,
  // Large screen / desktop (980 + (12 * 2)) <= container + gutters
  lg: $width-lg,
  // Extra large screen / wide desktop
  xl: $width-xl
) !default;

Media query mixins

Use media query mixins when you want to change CSS properties at a particular breakpoint. The media query mixin works by passing in a breakpoint value, such as breakpoint(md).

Media queries are scoped from each breakpoint and upwards. In the example below, the font size is 28px until the viewport size meets the lg breakpoint, from there upwards—including through the xl breakpoint—the font size will be 32px.

.styles {
  font-size: 28px;
  @include breakpoint(md) { font-size: 32px; }
}