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Canonical TAG, when to use it

What Information Have the Engines Provided About the Canonical URL Tag?

 

Carpe diem on any duplicate
content worries
: we now support a format that allows you to publicly specify
your preferred version of a URL. If your site has identical or vastly similar
content that’s accessible through multiple URLs, this format provides you with
more control over the URL returned in search results. It also helps to make sure
that properties such as link popularity are consolidated to your preferred
version.

Let’s take our old example of a site
selling Swedish fish
. Imagine that your preferred version of the URL and its
content looks like this:

http://www.example.com/product.php?item=swedish-fish

However, users (and Googlebot) can access
Swedish fish through multiple (not as simple) URLs. Even if the key information
on these URLs is the same as your preferred version, they may show slight
content variations due to things like sort parameters or category navigation:

http://www.example.com/product.php?item=swedish-fish&category=gummy-candy

Or they have completely identical content,
but with different URLs due to things such as a tracking parameters or a session
ID:

http://www.example.com/product.php?item=swedish-fish&trackingid=1234&sessionid=5678

Now, you can simply add this <link> tag to specify
your preferred version:

<link
rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish”
/>

inside the <head> section of the
duplicate content URLs:

http://www.example.com/product.php?item=swedish-fish&category=gummy-candy

http://www.example.com/product.php?item=swedish-fish&trackingid=1234&sessionid=5678

and
Google will understand that the duplicates all refer to the canonical URL: http://www.example.com/product.php?item=swedish-fish.
Additional URL properties, like PageRank and related signals, are transferred as
well.

This standard can be adopted by any search engine when crawling and
indexing your site.

Of course you may have more questions. Joachim Kupke, an
engineer from our Indexing Team, is here to provide us with the answers:

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.

Can I use a relative path to specify the canonical, such as <link rel=”canonical” href=”product.php?item=swedish-fish” />?
Yes, relative paths are recognized as expected with the <link> tag. Also, if you include a <base> link in your document, relative paths will resolve according to the base URL.

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.

from Yahoo!:

• The URL paths in the <link> tag can be absolute or relative, though we recommend using absolute paths to avoid any chance of errors.

• A <link> tag can only point to a canonical URL form within the same domain and not across domains. For example, a tag on http://test.example.com can point to a URL on http://www.example.com but not on http://yahoo.com or any other domain.

• The <link> tag will be treated similarly to a 301 redirect, in terms of transferring link references and other effects to the canonical form of the page.

• We will use the tag information as provided, but we’ll also use algorithmic mechanisms to avoid situations where we think the tag was not used as intended. For example, if the canonical form is non-existent, returns an error or a 404, or if the content on the source and target was substantially distinct and unique, the canonical link may be considered erroneous and deferred.

• The tag is transitive. That is, if URL A marks B as canonical, and B marks C as canonical, we’ll treat C as canonical for both A and B, though we will break infinite chains and other issues.

and from Live/MSN:

  • This tag will be interpreted as a hint by Live Search, not as a command. We’ll evaluate this in the context of all the other information we know about the website and try and make the best determination of the canonical URL. This will help us handle any potential implementation errors or abuse of this tag.
  • You can use relative or absolute URLs in the “href” attribute of the link tag.
  • The page and the URL in the “href” attribute must be on the same domain. For example, if the page is found on “http://mysite.com/default.aspx”, and the ”href” attribute in the link tag points to “http://mysite2.com”, the tag will be invalid and ignored.
    • However, the “href” attribute can point to a different subdomain. For example, if the page is found on “http://mysite.com/default.aspx” and the “href” attribute in the link tag points to “http://www.mysite.com”, the tag will be considered valid.
  • Live Search expects to implement support for this feature sometime in the near future.
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