Buck Anderson posted on August 06, 2009 09:10 :: 3291 Views
What is a canonical page? Why specify a canonical page?
A canonical page is the preferred version of a set of pages with highly similar content.
It's common for a site to have several pages listing the same set of products. For example, one page might display products sorted in alphabetical order, while other pages display the same products listed by price or by rating. For example:
http://www.example.com/product.aspx?item=swedish-fish&trackingid=1234567&sort=alpha
http://www.example.com/product.aspx?item=swedish-fish&trackingid=1234567&sort=price
If Google knows that these pages have the same content, we may index only one version for our search results. Our algorithms select the page we think best answers the user's query. Now, however, users can specify a canonical page to search engines by adding a <link> element with the attribute rel="canonical" to the <head> section of the non-canonical version of the page. Adding this link and attribute lets site owners identify sets of identical content and suggest to Google: "Of all these pages with identical content, this page is the most useful. Please prioritize it in search results."
How do I specify a canonical page?
To specify a canonical link to the page http://www.example.com/product.php?item=swedish-fish, create a <link> element as follows:
<link rel="canonical" href="http://www.example.com/product.aspx?item=swedish-fish"/>
Copy this link into the <head> section of all non-canonical versions of the page, such as http://www.example.com/product.php?item=swedish-fish&sort=price.
If you publish content on both http://www.example.com/product.php?item=swedish-fish and https://www.example.com/product.php?item=swedish-fish, you can specify the canonical version of the page. Create the <link> element:
<link rel="canonical" href="http://www.example.com/product.aspx?item=swedish-fish"/>
Add this link to the <head> section of https://www.example.com/product.aspx?item=swedish-fish.
Is rel="canonical" a suggestion or a directive?
This new option lets site owners suggest the version of a page that Google should treat as canonical. Google will take this into account, in conjunction with other signals, when determining which URL sets contain identical content, and calculating the most relevant of these pages to display in search results.
Can the link be relative or absolute?
The rel="canonical" attribute can be used with relative or absolute links, but we recommend using absolute links to minimize potential confusion or difficulties. If your document specifies a base link, any relative links will be relative to that base link.
Must the content on a set of pages be similar to the content on the canonical version?
Yes. The rel="canonical" attribute should be used only to specify the preferred version of many pages with identical content (although minor differences, such as sort order, are okay).
For instance, if a site has a set of pages for the same model of dance shoe, each varying only by the color of the shoe pictured, it may make sense to set the page highlighting the most popular color as the canonical version so that Google may be more likely to show that page in search results. However, rel="canonical" would not be appropriate if that same site simply wanted a gel insole page to rank higher than the shoe page.
What happens if rel="canonical" points to a non-existent page? Or if more than one page in a set is specified as the canonical version?
We'll do our best to algorithmically determine an appropriate canonical page, just as we've done in the past.
Can Google follow a chain of rel="canonical" designations?
Yes, to some extent, but to ensure optimal canonicalization, we strongly recommend that you update links to point to a single canonical page.
Can rel="canonical" be used to suggest a canonical url on a completely different domain?
No. To migrate to a completely different domain, permanent (301) redirects are more appropriate. Google currently will take canonicalization suggestions into account across subdomains (or within a domain), but not across domains. So site owners can specify a canonical page on www.example.com from a set of pages on example.com or help.example.com, but not on example-widgets.com.