Fans of TV shows such as Star Trek or Battlestar Galactica should be familiar with the term canon, as in canonical URLs.
The word canon means “official” and is actually a biblical term, referring to the books of the bible. Canon bible books were officially counted as part of the bible.
This is in contrast to other books, such as the Book of Enoch which did not get included in the official “register” of the bible (for the curious – those are collected in the “Apocrypha.”).
As for canonical URLs, these basically indicate which URL is the “official” URL for your website. For example, you might have a website with the following addresses:
Each of these different addresses all lead to your website’s homepage. For humans, it doesn’t really matter. However, for Google and other search engines, finding multiple URLs for the same page means that they start to assume that these things must of necessity be duplicate pages.
No Penalty But Problematic for Rankings
Contrary to what some may believe, Google doesn’t penalize websites that have the same content at multiple URLs. After all, the engineers behind the search engine understand that you may have multiple ways of reaching the same content. They therefore take this into account and don’t penalize you for it.
Instead, they only penalize websites which engage in “scraping,” grabbing all their content from other websites so that they can appear to have lots of original material, even though they really don’t.
However, as far as ranking are concerned, you want to make sure that one URL is the official and accepted URL for your website so that you can ensure that the search engines will always look at that one page and that one page will gain higher rankings.
Two Ways to Specify Canonical URLs
There are two basic ways to specify your canonical URLs. You can either choose to use a 301 permanent redirect which redirects everything to one URL (so for example, website.com redirects to www.website.com and website.com/home.asp also redirects there and so forth).
You would do this with your URL host by giving them all your alternate URLs and having them redirect traffic to your main URL. Your site visitors will then always be redirected to an address of your choosing, regardless of which version of the URL they entered.
The big advantage of a 301 redirect is that it keeps things neat for the search engines and makes it less confusing for your visitors. The downside however is that it makes it harder to track which URL is getting the most hits. Therefore, you may want to instead use the canonical URL tag in your website.
Canonical URL Tags
The alternative is to use canonical URL tags, which all the major search engines began supporting about two years ago. Basically, what you would do is to insert a single line into the header of your website page:
<link rel=”canonical” href=”http://www.website.com” />
This simply tells Google and other search engines that the page they are visiting is a copy of the page at your main website and to treat it as such.
The disadvantage of this however is that you can’t do a redirect from different URLs. This means that if you own similar URLs (i.e. website.com and websites.com) and want to redirect traffic from the latter to the former, you must use a 301 redirect instead of this tag.