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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. Google and other search engines now do not give credit to reciprocal linking as it does not indicate genuine link popularity.
Resource links are a category of links, which can be either one-way or two-way, usually referenced as "Resources" or "Information" in navbars, but sometimes, especially in the early, less compartmentalized years of the Web, simply called "links". Basically, they are hyperlinks to a website or a specific webpage containing content believed to be beneficial, useful and relevant to visitors of the site establishing the link.
In recent years, resource links have grown in importance because most major search engines have made it plain that—in Google's words— "quantity, quality, and relevance of links count towards your rating".
Search engines measure a website’s value and relevance by analyzing the links to the site from other websites. The resulting “link popularity” is a measure of the number and quality of links to your website. It is an integral part of a website’s ranking in search engines. Search engines examine each of the links to your website to determine its value. Although every link to a website is a vote in its favor, not all votes are counted equally. A website with similar subject matter to the website receiving the inbound link carries more weight than an unrelated site, and a well-regarded site (such as a University) has higher link quality than an unknown or disreputable website.
The text of links helps search engines categorize your website. If your business is a mattress warehouse, a link to it titled “Good Value Mattresses” is helpful, while “Sweet Skateboard Wheels” would be less useful. The engines' insistence on resource links being relevant and beneficial developed because many artificial link building methods were employed solely to "spam" search-engines, i.e. to "fool" the engines' algorithms into awarding the sites employing these unethical devices undeservedly high page ranks and/or return positions.
Despite cautioning site developers (again quoting from Google) to avoid "'free-for-all' links, link popularity schemes, or submitting your site to thousands of search engines (because) these are typically useless exercises that don't affect your ranking in the results of the major search engines -- at least, not in a way you would likely consider to be positive," most major engines have deployed technology designed to "red flag" and potentially penalize sites employing such practices.
Forum signature linking is a technique used to build backlinks to a website. This is the process of using forum communities that allow outbound hyperlinks in a member's signature. This can be a fast method to build up inbound links to a website Search Engine Optimization value.
Blog comments 
Leaving a comment on a blog can result in a relevant do-follow link to the individual's website. Most of the time, however, leaving a comment on a blog turns into a no-follow link, which is almost useless in the eyes of search engines, such as Google and Yahoo! Search. On the other hand, most blog comments get clicked on by the readers of the blog if the comment is well-thought-out and pertains to the discussion of the other commenters and the post on the blog.
Website directories are lists of links to websites, which are sorted into categories. Website owners can submit their site to many of these directories. Some directories accept payment for listing in their directory, while others are free.
Social bookmarks 
Social bookmarking is a way of saving and categorizing web pages in a public location, on the web. Because bookmarks have anchor text and are shared and stored publicly, they are scanned by search engine crawlers and have search engine optimization value.
See also 
- Backlink: incoming links
- Deep linking: linking directly to a page within another website.
- Inline linking: linking directly to content within another website.
- Link doping: The practice and effects of embedding a large number of hyperlinks on a website in exchange for return links, especially when used to inflate the apparent popularity of the website.
- Page Rank
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