President of WebFX. Bill has over 25 years of experience in the Internet marketing industry specializing in SEO, UX, information architecture, marketing automation and more. William's background in scientific computing and education from Shippensburg and MIT provided the foundation for MarketingCloudFX and other key research and development projects at WebFX.

Assigning the rel="nofollow" attribute to a link informs search engines that the link is commercial in nature, or that it’s a link you don’t endorse.

Links that have the the rel="nofollow" attribute are often called nofollow links.

Just so we’re on the same page, here’s an example of a link that uses the rel="nofollow" attribute:

<a rel="nofollow" href="http://example.com">nofollow link</a>

The concept behind nofollow links came about in 2005, as a way of discouraging comment spam in blogs.

You might already know this if you’re familiar with SEO: One of the factors search engines take into account when ranking a website is the number of other websites linking to it (referred to as the site’s inbound links).

If there are many links from other sites pointing to your site, and those other sites are reputable, then there’s a good chance your site is also reputable.

To artificially boost their clients’ search engine rankings, spammers will try to place as many links as they can on other websites. An easy target for spammers are the “comments” sections of reputable blogs.

To help address the spam issue, the rel="nofollow" attribute was introduced.

The thinking went like this: If comment spammers didn’t get credit for their nofollow links, then they’d be less likely to waste their time posting those links on blogs.

Today, the purpose of rel="nofollow" has evolved slightly. It’s now also supposed to be used on links that are commercial/for-profit in nature. Links that fall into this category are:

paid links

banner ads

sponsored content

text link ads

affiliate links

The rel="nofollow" attribute started off as a microformat. However, the HTML attribute is now in the latest W3C HTML specification, in section 4.8.4.7 Link type “nofollow”.

Here’s how the attribute should be used in accordance to specs:

The nofollow keyword indicates that the link is not endorsed by the original author or publisher of the page, or that the link to the referenced document was included primarily because of a commercial relationship between people affiliated with the two pages. 4.8 Links — HTML5

So, When Should You Use rel=”nofollow”?

Based on the specs, the two cases in which you should use rel="nofollow" are:

When the link is something you don’t/can’t endorse When the link is primarily commercial in nature

Let’s talk about these two cases.

Links You Don’t Endorse

A typical example of a link you can’t endorse is one that was placed on your site by someone else.

For example, if you have a blog, the comments on your blog are posted by your readers. Those comments often contain links. It’s not practical to review each and every link commenters post on your site. Using the rel="nofollow" attribute in the “comments” section of your blog essentially tells search engines, “Hey, I haven’t reviewed, and therefore can’t vouch for, any of the links you find here.”

That’s why, by default, WordPress (1.5 and above) automatically sets the rel="nofollow" attribute on user-generated links in the “comments” sections of WordPress-powered sites.

Wikipedia uses the rel="nofollow" attribute on all external links. Wikipedia can be edited by anyone. And because of the sheer volume of content on the site, it’s not practical for Wikipedia to verify, review, and endorse each and every external link posted on their site. So Wikipedia decided to categorically assign the rel="nofollow" attribute on all external links, much like how WordPress categorically assigns the attribute on all external links it finds in the “comments” section.

On Wikipedia, notice how an external link that goes to w3.org — which you and I know is a pretty reputable site — is a nofollow link.

With Chrome DevTools, we can see that external links in Wikipedia have rel="nofollow" attributes.

On Reddit, new link submissions are nofollow.

What’s interesting with Reddit’s implementation is that when an external link has been voted to the front page of the site, its rel="nofollow" attribute is removed.

Reddit’s system of assigning rel="nofollow" attributes is clever because it’s democratic. Reddit makes the hypothesis that if a submitted link gets the required votes needed to propel itself to the site’s front page, then it must mean enough people have reviewed the trustworthiness of the link, and thus the link’s rel="nofollow" attribute can be safely dropped.

Links That Are Commercial in Nature (Paid Links)

When a link exists on your site primarily for profit, it should be assigned a rel="nofollow" attribute.

Simply put: If you publish paid advertisements on your site — banner ads, text links, sponsored content, affiliate links, etc. — the links going to your advertisers’ sites should be nofollow.

This is one way to discourage any site from trying to buy its way into better search rankings by paying other sites for links.

Why Use rel=”nofollow”?

The rel="nofollow" attribute benefits third-party services most. Search engines, for example, can use the information we give them about links on our site as a clue that certain links might be low-quality, or commercial in nature, or both. They can then use this knowledge to improve the quality of their search results for their users.

So what’s in it for us?

By disclosing which links on our site have been paid for and which ones haven’t been reviewed, we can reduce the chances of search engines penalizing our site for publishing low-quality external links.

It’s reasonable to assume that by diligently assigning the rel="nofollow" attribute on our unverified, untrusted, and paid links we can improve our website’s SEO, and our website’s trustworthiness as a source of quality content.

Learn More About Nofollow Links

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Jacob Gube is the founder of Six Revisions. He’s a front-end developer. Connect with him on Twitter.