What is a Canonical Tag?
The canonical tag helps Google and other search engines understand which pages have original content and which pages are duplicates. This establishes the proper authority and ranking of the original content, without penalizing it for having a duplicate page elsewhere.
The canonical is placed in the <head> section of the original page as well as on the page with the duplicate content. It will look like:
Why are canonical tags important?
Although you should avoid duplicate content as a general rule, there are some instances where it can actually be useful. Typically, this involves content syndication, where you promote parts of your published content on other pages to help bring more readers back to your original site. The canonical can be an important part of letting Google know that the second page is a duplicate and that the first page is an original. Without the tag, Google might penalized one or both sites for duplicate content. The search engine wants to provide its users with the optimal user experience, and it knows that duplicate content detract from that experience.
What do canonical tags do?
Kirill Kronrod, along with BrightEdge technology, did a case study of a small site that began with improperly configured tags and examined what happened when the tags were fixed. The site saw improvements in the impressions, the number of ranked keywords, the rank of those keywords and your traffic rates. The results happened quickly and were immediately obvious, indicating the importance of the tag to tell Google which pages have the original content.
The canonical tag is an important part of letting Google know which site is original and should be ranked as such and which pages should be disregarded. Without this tag, Google is left guessing, which lowers the rankings of your site. If you have any duplicate content on the web, make sure to use the canonical to avoid hits to your ranking.