Any Markdown file that contains a YAML frontmatter block will be processed by [gray-matter](https://github.com/jonschlinkert/gray-matter). The frontmatter must be at the top of the Markdown file, and must take the form of valid YAML set between triple-dashed lines. Example:
Between the triple-dashed lines, you can set [predefined variables](/reference/frontmatter-config), or even create custom ones of your own. These variables can be used via the special <code>$frontmatter</code> variable.