The Complete Guide to Canonical Tags

This is a complete guide to Canonical Tags - which ideally all websites should have in place.

History

Canonical tags were introduced to the world by Google on XX/XX/XXXX with the aim of assisting them with removing duplicate content - which at that time was a big technical challenge for the Search Giant.

What issues do Canonical Tags resolve?

Your own website serving the same content on multiple pages. E.g. a product page being available on more than 1 URL, because its been placed in multiple shopping categories.
A more basic example could be UTM Tracking, which is used in e-mail and other marketing campaigns to segment traffic in Google Analytics.

E.g. https://www.anthonyshapley.com/?utm_source=newsletter&utm_medium=email&utm_campaign=first_release, you’ll see we have some extra information in the link, which passes information to Google Analytics about the origins of traffic that would have otherwise been recorded as direct. You’ll notice https://www.anthonyshapley.com/?utm_source=newsletter&utm_medium=email&utm_campaign=first_release and https://www.anthonyshapley.com/ are identical.

If too many people linked to the UTM tracked version, without a canonical tag - this could cause Google or other search engines to become confused about what version of the page they should be showing to users.

What it won't fix:

A website copying your text and republishing it. A canonical tag will not help with this, unless you control the website copying and republishing, in which case we advise you simply stop!

Implementing Canonical Tags - Source Code

Implementing the canonical tag is very simple for developers, lets use an example of my Contact Us page. We have the URL https://www.anthonyshapley.com/contact-us and we also have the URL https://www.anthonyshapley.com/contact-us?utm_source=newsletter, which is from my most recent mailshot. This mail shot has an online version to which is linking to the UTM version, so Google may index both.

In this instance, we would look to implement a canonical tag in the <head></head> of the page, the code would look like this:

<link rel="canonical" href=“https://www.anthonyshapley.com/contact-us“ />

Implementing Canonical Tags - HTTP Headers

How to check for Canonicalisation Issues inside your website

Canonicalisation: Reliance on the canonical tag to prevent duplicate content. This is not ideal. Canonical tags should be treated as a safe guard not a ‘solution’.

Often developers don’t consider SEO implications of how a site functions when its being written. Therefore it can be beneficial to do a full sweep of your existing site - no matter how large to determine if it has canonicalisation issues.

Checking an entire site for these issues can be difficult without the appropriate tools and processing power. A site crawl can be done using either ScreamingFrog or SiteBulb, personally we like ScreamingFrog.

ScreamingFrog has a predefined report called “Canonicalised” under “Directives”. This lists all examples of where an internal link is not linking to the Canonical version of a page.

Why shouldn't I rely on the canonical tag?

If too many of your internal links point to the non Canonical version of a URL, Google and other Search Engines may begin to lose trust in your Canonical Tag configuration.
You lose PageRank by not linking to the Canonical URL, if you link to 1,000 different versions of the same URL, incorporating different query strings - your page will not be performing at its best, because some PageRank is lost through reliance on Canonical Tags.

Things not to do with Canonical Tags

Over the years we’ve seen some very creative uses for these tags, with unclear aims. We’ve collated our experiences into a simple list of things not to do. In all instances the reason not to do the following things is the same. It will either prevent proper crawling of your website or force Google and other search engines to completely ignore / not trust your implementation.

Canonical Tags are not a directive, so Google has never obligated itself to use them, if its algorithms decide the implementation is wrong.

Pagination; Most common of all errors is clustering all of a sites pagination back to Page 1, don’t do this - it will prevent proper crawling of your products, or most likely search engines will just completely ignore all your canonical tags. Instead, if you don’t want your pagination indexed look here and here for proper solutions
Grouping Similar Content; Canonical tags are designed for preventing exact duplicate pages being indexed together - they should not be used for clustering batches of similar content together. Instead look to build better more in-depth pages and 301 redirect the similar content to a single solid resource.
Often we recommend using a Meta Robots tag to exclude filters and pagination from being indexed by Search Engines. The aim being to only have for instance a single URL for a category page indexed. This stops competing URLS with very close content. However we strongly recommend against doing things like appending a Meta Robots tag to noindex a page, and then also canonicalising that URL back to the category page.

That pretty much covers the important stuff, you can read more regarding canonical tags at the following locations:

Need your Canonical Tag implementation checked? Call us on +44 (0)1626 210032 or e-mail us.

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