Long time no blog! I hope you all had a good festive season. I thought I would kick off the new year with a technical post, as Google announced cross-domain support of the Canonical tag last month (worth reading for the explanations of when you might want to use it and how to implement).
You may remember from my earlier post on the canonical tag, that it is a way of telling the search engines the “master” address of a page, when multiple addresses for the same content might exist. Why would you have multiple addresses (URLs) for a page, you might wonder? Well, how about a product list on an e-commerce website with options for ordering the products alphabetically, by price or by manufacturer? It’s likely that the URL will be different in some way for each version of the list, even though its contents are actually the same. That means that a search engine will index all three versions (or possibly six if you have reverse-order options too).
Why the problem? Well, you probably want visitors to see that list in a certain order the first time they visit, let’s say ordered by price, cheapest first. If Google has all six versions of that page in its database, what’s to say it won’t link to your price: descending (i.e. most expensive first) list from its search results? That might make you look expensive and put off potential buyers.
The other issue is link juice – with multiple addresses for the same page, you might have some links to one URL, some to another, all essentially to the same page but for Google, they are different pages. That means the link juice is being split between those different versions of the page. So, using the rel=canonical tag, you can tell Google what the master version of the page is and that therefore, all link juice should be applied to that version and that’s the one that should appear in search results.
This is what it looks like:
<link rel="canonical" href="http://www.website.com/category/product-page">
It goes in the <head> section of each version of the page, so in the product list example, your page would contain the above code regardless of what version is being displayed at the time. This would probably be done automatically by your content management system, so that when a different category of products is being displayed, the canonical tag references the correct category/product list, because it’s likely the same page template is used for all categories.
In effect, the canonical tag works like a 301 redirect, but without you having to mess around with server settings. What changed in December is that now, you can make cross-domain (i.e. cross-website) canonical tags, when before, you could only use it within one domain. So, even those of you with problematic servers (for example, you’re on shared Windows hosting without access to IIS Admin), you can now create “301”-style redirects, avoiding duplicate content issues.
As noted by Rand, there is no problem having the canonical tag in the “master” page.