Bi-directional Annotation

Bi-directional Annotation is a phrase coined by Google to describe certain code that they request webmasters use to link a mobile version of a web page with the mobile version of the same web page. The code is designed for sites that have a desktop version of the page on the ‘www’ subdomain and the mobile version of the page on the ‘m’ subdomain. For both versions of the page, code placed in the <head> tag of both pages. The mobile version of the code, which goes in the mobile <head> tag points of the corresponding desktop version of the page. It is a rel=canonical tag. The desktop version of the code, which goes in the desktop <head> tag points to the mobile version of the page, and it is a rel=canonical tag.

Mobile Page Code (http://m.example.com/page-1): <link rel=”canonical” href=”http://www.example.com/page-1″>
Desktop Page Code (http://www.example.com/page-1): <link rel=”alternate” media=”only screen and (max-width: 640px)”
href=”http://m.example.com/page-1″>