What are a Rel Canonical Tags and When Should I Use Them?
A rel canonical tag is a special tag that is inserted into the header of your HTML that helps communicate to search engine bots the relationship of that piece of content to others on your site. Canonical tags tell the search engine bots which pieces of content are the original or primary ones and which are duplicates. This way the bot will pass over the duplicates and only index and give link credit to the primary piece.
Here's a longer explination by Matt Cutts of Google back in 2009:
Here are some very common times canonical tags are used:
- Multiple Category Views which have the same results. If you have an ecommerce website that offered multiple ways to view products based on categories (i.e. blue shirts, mens shirts, fitted shirts) and these offer the same results. You will want to use a canonical tag on the categories. I would not suggest no-index because it can hurt your SEO efforts.
- Mobile websites on a sub-domain or sub-directory. If you have a separate mobile template website that is on a sub-domain or sub-directory you will have to use canonical tags to help communicate with the search engine bot that they are similar pieces of content. Additionally, on the desktop version, you need to include a rel alternate tag that points back to the mobile piece of content.
- Tags in Blogs. If you have a blog and have a tagging system to help organize the blogs, there's a good chance you are running into duplicate content issues. If the same 3 blogs are tagged with both "SEO" and "Inbound Marketing" then they can show up in both tag categories which creates duplicate content. You can use rel canonical tags to help solve these duplicate content issues and explain to bots that this is a category view.
Here are some videos answering some common canonical questions:
- If you're looking for a bit more detailed and technical explination, please check out this blog post by Anything-Digital
These are just some examples of when you can use canonical tags. Additionally, there are other tags that are preferred such as pagination tags and rel lang tags for multilingual sites. If you are not sure exactly how to use these tags or have questions, feel free to post in the comments below.