What is a Canonical Tag?
A canonical tag looks like this: <link rel=”canonical” href=http://www.example.com/” /> this should be inserted into the head section of the HTML of a page within a website. (See below)
What Does it Look like?
Why is it Important?
Many SEO technicians suggest a canonical tag appears on every single page of a website for ‘good practice’ – but why?
The main functionality of a canonical tag is to signal to Google which page you consider to be the most important from a group of pages which can have very similar content.
This can be very useful when you have duplicate pages on your website e.g. the homepage or you have a number of similar product pages which produce multiple URLs on different sort filters or tracking parameters.
AND WE ALL KNOW DUPLICATE CONTENT IS BAD
Examples of when to use it:
Often on websites the homepage and folder levels will be duplicated. This is because the page and its content can be found in another folder for example
http://www.homepage.com/index.html
Is the same as
http://www.homepage.com/
When you’re a webmaster or SEO it is important to know how to look for these. The duplicate homepage at folder level can be found by adding a range of different default naming conventions to the top level domain, depending on which server your website is hosted.
There are 2 main server
There are 2 main servers
Microsoft IIS
/Default.aspx /Index.asp /Home.asp /Index.htm
Apache
/Index.html /Index.php /Default.php /Home.php
Finding Duplicate Content through Google:
Google will automatically omit web pages from its search results that it considers to be duplicated. However this doesn’t mean that it cannot see them which will be causing damage to your SEO.
To find out exactly what Google considers to be duplicate content, we can use a ‘site colon’ to identify the pages.
For example: ‘site:www.riverisland.com’
If we scroll then through the results to the end this, is what we will come across:
So take a note of the last URL in the SERP’s and then scroll through to this. When you find it (it should be around the same position) any URLs after this will then be shown as being very similar in the search engines eyes and may need to have canonical tags inserted.
Sorting and Filtering
Another example where to use the Canonical tag is when a website features a sort filter. This is very common on e-commerce websites:
The top level of the category forms on this URL
http://www.riverisland.com/Online/men/t-shirts–vests
However when we sort it by ‘latest in stock’ the URL forms like this
Example 1
and if we filtered it by prices low to high the URL changes to this:
Example 2
Now theoretically much of the content on these pages is similar, so in order to signal that we would like http://www.riverisland.com/Online/men/t-shirts–vests to be considered the main page for all of these variations what do we need? Yes you guessed it…
<link rel=”canonical” href=”http://www.riverisland.com/Online/men/t-shirts–vests“ />
We need the canonical tag on Example 1 and Example 2 and any other example of filtering which can be found throughout the website.
UPPERCASE and lowercase
*One more thing to take into account is issues with uppercase and lowercase. For example:
http://www.myclothes.com/Online/Mens/
http://www.myclothes.com/online/mens/
If a page resolves on both of these this can also be classed as duplicate content and will need to be redirected to the preferred URL or have a canonical tag present.
Duplicate Content Caused by URL Parameters, Like Session IDs or Tracking IDs
Tracking parameters exist to identify the different sources through which traffic is driven to a given page. Session IDs exist to identify a specific user. Whilst both are extremely useful, they can cause duplicate content. If a user links to the URL
http://www.prodo.com/?utm_source=newsletter4&utm_medium=email&utm_campaign=prom1
It could then appear in the eyes of Google as a URL.
A canonical tag will need to be applied to this to avoid the issue of duplicate content.
Final Word
The Canonical tag should be used in situations where duplicate content may be an issue due to similar pages within your website, duplicate pages where they cannot be redirected and for pages created through tracking parameters just like the examples previously mentioned.
It is important to remember however, that even though Google strongly adheres to the canonical tag, it is always better to redirect when it is safe and possible.
If you have any questions about the canonical tag and could use it for your website, feel free to comment in the space below and Prodo will happily respond.
