- **Breakpoints are the building blocks of responsive design.** Use them to control when your layout can be adapted at a particular viewport or device size.
- **Use media queries to architect your CSS by breakpoint.** Media queries are a feature of CSS that allow you to conditionally apply styles based on a set of browser and operating system parameters. We most commonly use `min-width` in our media queries.
- **Mobile first, responsive design is the goal.** Bootstrap's CSS aims to apply the bare minimum of styles to make a layout work at the smallest breakpoint, and then layers on styles to adjust that design for larger devices. This optimizes your CSS, improves rendering time, and provides a great experience for your visitors.
## Available breakpoints
Bootstrap includes six default breakpoints, sometimes referred to as _grid tiers_, for building responsively. These breakpoints can be customized if you're using our source Sass files.
{{<bs-table"table">}}
| Breakpoint | Class infix | Dimensions |
| --- | --- | --- |
| Extra small | <em>None</em> |<576px |
| Small | `sm` | ≥576px |
| Medium | `md` | ≥768px |
| Large | `lg` | ≥992px |
| Extra large | `xl` | ≥1200px |
| Extra extra large | `xxl` | ≥1400px |
{{</bs-table>}}
Each breakpoint was chosen to comfortably hold containers whose widths are multiples of 12. Breakpoints are also representative of a subset of common device sizes and viewport dimensions—they don't specifically target every use case or device. Instead, the ranges provide a strong and consistent foundation to build on for nearly any device.
These breakpoints are customizable via Sass—you'll find them in a Sass map in our `_variables.scss` stylesheet.
For more information and examples on how to modify our Sass maps and variables, please refer to [the CSS section of the Grid documentation]({{< docsref "/layout/grid#css" >}}).
Since Bootstrap is developed to be mobile first, we use a handful of [media queries](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_media_queries/Using_media_queries) to create sensible breakpoints for our layouts and interfaces. These breakpoints are mostly based on minimum viewport widths and allow us to scale up elements as the viewport changes.