canonical:

basic definition: reduced to the simplest and most significant form possible without loss of generality; “a basic story line”; “a canonical syllable pattern”

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.php?item=swedish-fish&trackingid=1234567&sort=alpha&sessionid=5678asfasdfasfd
  • http://www.example.com/product.php?item=swedish-fish&trackingid=1234567&sort=price&sessionid=5678asfasdfasfd

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.”

WordPress has traditionally been very lenient in the URLs that it will accept.  So, what’s the problem with this? The URLs are all showing the exact same content, so why should it matter? Well, search engines can’t assume that all of these alternative URLs represent the same resource. So they don’t automatically get condensed into a single resource. As a result, you can actually end up competing against yourself in search engine rankings. So to avoid confusing search engines and to consolidate your rankings for your content, there should only be one URL for a resource. We call this URL the canonical URL. Canonical means “standard” or “authoritative.” It’s the one that WordPress generates, and it’s the one that you want everyone to use.

How do I specify a canonical page?

  1. 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.php?item=swedish-fish"/>
  2. http://yoast.com/wordpress/canonical/ – This plugin helps you solve duplicate content issues on your site. It’s not the cure for all evil, but it will make it easier by preventing tagged URL’s from being indexed.