Matt Cutts, Google’s head of search spam, answers a question about linking domains together in his latest Webmaster Help video where a user writes in to ask:
Is there any way Google identifies “sister” websites? For example, relationships between ebay.co.uk and ebay.com? Does linking from one to the other is taken as paid or unnatural? And I’m strictly talking about good, genuine ccTLDs for businesses :)
Matt says Google does the best they can to interpret relationships between websites, but it would also be helpful if users could tell Google a little bit more about what their sites are so Google can return the right content to the right users regardless of which country they’re coming from.
The best way to help Google with this, Matt says, is to use something called hreflang. You can use this inside of a webpage by using markup code that tells Google where to find other variations of the same domain.
Google tries to interpret where users are coming from and show them the best version of a page according to their language, location, etc. But linking pages together with hreflang would help Google do their job of returning the correct page more effectively.
For the most part, Matt says not to worry about Google viewing this kind of linking as paid or unnatural, but he still suggests to make it look as organic as possible. For example, stuffing the footer with dozens of different links to similar domains would start to look a bit spammy.
To hear Matt’s full response in his own words, please see the video below: