Canonical URL Tag
The tag is part of the HTML header on a web page, the same section you'd find the Title attribute and Meta Description tag. In fact, this tag isn't new, but like nofollow, simply uses a new rel parameter.
link href="https://moz.com/blog" rel="canonical"
This would tell Yahoo!, Live & Google that the page in question should be treated as though it were a copy of the URL moz.com/blog and that all of the link & content metrics the engines apply should technically flow back to that URL.
The Canonical URL tag attribute is similar in many ways to a 301 redirect from an SEO perspective. In essence, you're telling the engines that multiple pages should be considered as one (which a 301 does), without actually redirecting visitors to the new URL (often saving your dev staff considerable heartache). There are some differences, though:
- Whereas a 301 redirect re-points all traffic (bots and human visitors), the Canonical URL tag is just for engines, meaning you can still separately track visitors to the unique URL versions.
- A 301 is a much stronger signal that multiple pages have a single, canonical source. While the engines are certainly planning to support this new tag and trust the intent of site owners, there will be limitations. Content analysis and other algorithmic metrics will be applied to ensure that a site owner hasn't mistakenly or manipulatively applied the tag, and we certainly expect to see mistaken use of the tag, resulting in the engines maintaining those separate URLs in their indices (meaning site owners would experience the same problems noted below).
- 301s carry cross-domain functionality, meaning you can redirect a page at domain1.com to domain2.com and carry over those search engine metrics. This is NOT THE CASE with the Canonical URL tag, which operates exclusively on a single root domain (it will carry over across subfolders and subdomains).
How, When & Where Should SEOs Use This Tag?
In the past, many sites have encountered issues with multiple versions of the same content on different URLs. This creates three big problems:
- Search engines don't know which version(s) to include/exclude from their indices
- Search engines don't know whether to direct the link metrics (trust, authority, anchor text, link juice, etc.) to one page, or keep it separated between multiple versions
- Search engines don't know which version(s) to rank for query results
When this happens, site owners suffer rankings and traffic losses and engines suffer lowered relevancy. Thus, in order to fix these problems, we, as SEOs and webmasters, can start applying the new Canonical URL tag whenever any of the following scenarios arise:
- my category url systems create multiple versions of the same page (www.widgets.com/blue-widgets?color=blue
- my print-only urls create multiple versions of the same page (www.widgets.com/print/blue-widgets
- my session id urls create multiple versions of the same page (www.widgets.com/blue-widgets?sessid=123
Is rel="canonical" a hint or a directive?
It's a hint that we honor strongly. We'll take your preference into account, in conjunction with other signals, when calculating the most relevant page to display in search results.
Is it okay if the canonical is not an exact duplicate of the content?
We allow slight differences, e.g., in the sort order of a table of products. We also recognize that we may crawl the canonical and the duplicate pages at different points in time, so we may occasionally see different versions of your content. All of that is okay with us.
What if the rel="canonical" returns a 404?
We'll continue to index your content and use a heuristic to find a canonical, but we recommend that you specify existent URLs as canonicals.
What if the rel="canonical" hasn't yet been indexed?
Like all public content on the web, we strive to discover and crawl a designated canonical URL quickly. As soon as we index it, we'll immediately reconsider the rel="canonical" hint.
Can rel="canonical" be a redirect?
Yes, you can specify a URL that redirects as a canonical URL. Google will then process the redirect as usual and try to index it.
What if I have contradictory rel="canonical" designations?
Our algorithm is lenient: We can follow canonical chains, but we strongly recommend that you update links to point to a single canonical page to ensure optimal canonicalization results.