L ots of discussion about canonicalization and canonical URLs lately. Many sites have similar page content in their websites. In other words, a canonical URL is the URL that you want visitors to see.The easiest way to avoid this is to let the Search engines and the users know which is your “preferred URL” a.k.a canonical URL. And in your case this could be the wrong version (page) and you could miss a lot of potential traffic. This is what we call a canonical issue.
Watch Matt Cutts of Google introduces the canonical link element below:
Source: Google Webmaster Central
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:
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 element with the attribute rel=”canonical” to thesection 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 URL?
You can specify a canonical URL in two ways:
Add a rel=”canonical” link to the section of the non-canonical version of each HTML page.
To specify a canonical link to the page http://www.example.com/product.php?item=swedish-fish, create a element as follows:
Copy this link into thes ection 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 element:
Add this link to the section of https://www.example.com/product.php?item=swedish-fish. Indicate the canonical version of a URL by responding with the Link rel=”canonical” HTTP header. Adding rel=”canonical” to the head section of a page is useful for HTML content, but it can’t be used for PDFs and other file types indexed by Google Web Search. In these cases you can indicate a canonical URL by responding with the Link rel=”canonical” HTTP header, like this (note that to use this option, you’ll need to be able to configure your server):
Link: <http://www.example.com/downloads/white-paper.pdf>; rel=”canonical”
Google currently supports these link header elements for Web Search only.
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?
rel=”canonical” 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?
There are situations where it’s not easily possible to set up redirects. This could be the case when you need to migrate to a new domain name using a web server that cannot create server-side redirects. In this case, you can use the rel=”canonical” link element to specify the exact URL of the domain preferred for indexing. While the rel=”canonical” link element is seen as a hint and not an absolute directive, we do try to follow it where possible.