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1. Sites that use responsive web design, i.e. sites that serve all devices on the same set of URLs, with each URL serving the same HTML to all devices and using just CSS to change how the page is rendered on the device. This is Google's recommended configuration.
2. Sites that dynamically serve all devices on the same set of URLs, but each URL serves different HTML (and CSS) depending on whether the user agent is a desktop or a mobile device.
3. Sites that have separate mobile and desktop URLs.
As it is not immediately apparent in this setup that the site alters the HTML for mobile user agents (the mobile content is "hidden"), we recommend that the server send a hint to request that Googlebot-Mobile should crawl the page, too, and thus discover the mobile content. This hint is implemented using the Vary HTTP header.