A reciprocal link is a mutual link between two objects, commonly between two websites, to ensure mutual traffic. For example, Alice and Bob have websites. If Bob's website links to Alice's website and Alice's website links to Bob's website, the websites are reciprocally linked. Website owners often submit their sites to reciprocal link exchange directories in order to achieve higher rankings in the search engines. Reciprocal linking between websites is no longer an important part of the search engine optimization process. In 2005, with their Jagger 2 update, Google stopped giving credit to reciprocal links as it does not indicate genuine link popularity.
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Blog and forum comments
User-generated content such as blog and forum comments with links can drive valuable referral traffic if it's well-thought-out and pertains to the discussion of the post on the blog. However, these links almost always contain the nofollow or the newer ugc attribute which signal that Google shouldn't take these into its ranking considerations.