Typically when questions are asked about cross linking they're posed from a search engine standpoint...."how much is too much" or " how much can I link and stay under the radar"...
I always offer the same answer - do it within reason, as a marketing tactic and do it so it doesn't look *stupid*. Not very technical answers but pretty much the way it is.
From what I've seen, cross linking comes in two flavors: the first is the owner with multiple sites who wants to cross promote. He links to and from every page of every site by adding links down at the bottom of the pages.
The other scenario is the site owner who creates multiple sites for the sole purpose of cross linking them which some construe as link farming. (I have an issue with that, I say it can be a smart tactic… but that's for another time.) They too cross link each page from each site to the other, creating a real link web.
Either scenario is overkill IMO as people tend not to look at links found on every page of the site especially if they're way down at the bottom and in small font. I refer to these links as "fat links" or "footer links" since they're all bunched together in rows on top of each other. My eyes have learned to discount their existence since they're hard to read and usually non related.
If my eyes discount them, stands to reason a search bot does the same. Lots of links on the bottom of a page? Probably spam, move along little botdoggie.....
Note I said discount and not penalize. Hard penalties outside of an update are few and far between in cases like this, I've never heard of or known a site to be removed from the index for having bad taste. Cloaking yes.....hidden text yes - fat links - no.
The idea behind cross linking should be about cross promotion - both for visitors, your brand and the engines. Can you cross link non topic related sites? Sure! Even non complimentary sites can cross link, just create a resource page or highlight an area that explains why you're linking out. It gives your visitor a clue and adds optimized content to the site. I'd stay away from linking to and from every page.
Here is the quote from a Google rep I use in all my presentations, read closely:
I don't buy the idea search engines automatically discount links from cross linked sites because they're on the same IP with a number of others or they're cross linking with sites outside their industry. SE don't tell us how to handle our marketing efforts unless we're creating them to manipulate rankings. And when you cross link for promotional purposes, explaining your links and resources isn't manipulation, it's just good customer service and naviagtional options.
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