# Using margin: auto; for centering

In addition to specifying explicit values for `margin`, we can use the keyword `auto`.

```css
.centered {
  margin-left: auto;
  margin-right: auto;
}
```

The browser will seek to fill the maximum available space by applying the same margin on the left and right sides of the element. This has the side-effect of centering the element: neat!

There are two caveats:

* It only works for horizontal margin
* It only works on elements with an explicit width

With *grid* and *flexbox*, we have other ways of centering elements, but the `margin: auto` trick is especially useful because it can be selectively applied to a single element on a page.