Canonicalization is a way of “telling” search engines which URL they should crawl in case there are multiple variants of the same page. This helps search engines find the pages you want them to and can eliminate duplicate content issues. Without a canonical page, crawlers would crawl and index all available versions identifying them as duplicates and potentially hurting your crawl budget.
Why is Having Duplicate Content a Problem?
There are a couple of problems in terms of SEO that duplicate pages can cause.
Search engines don’t like to see duplicate content on websites and may penalize you if they find identical or very similar content on multiple pages that they crawl. By using canonical tags correctly, you can avoid this by telling bots that certain URLs are variants of a single canonical URL which is the only version that should be crawled.
Further, if your website has a lot of very similar pages (for example a product available in several different colors, or some other minor difference), crawlers will spend more time indexing each of them, instead of URLs with unique content. As a result, you may find your pages getting indexed at a slower rate.
How to Use Canonical Tags
The canonical tag (rel=”canonical”) needs to be implemented in your webpage’s code, followed by href=”the canonical URL“. When a crawler sees this code on a page, they will immediately know what URL you want them to crawl, ignoring the others.
In order to properly canonicalize your pages, there are a couple of things to keep in mind.
Consistency
Always have a single canonical URL for all duplicate or near-duplicate pages. What this means is that you should have the same canonical tag on all these web pages. Otherwise, search engines will make the choice for you, or even ignore the tag altogether.
Non-Canonical Pages Won’t Rank
This is probably obvious, but it’s important to keep in mind when canonicalizing near-duplicate pages. Canonical tags are used to prevent very similar pages (both in terms of content and purpose) from being crawled by pointing out one canonical URL. If you still want both pages to be crawled and indexed, while avoiding duplicate content issues, you’ll need to produce some unique content for them.
On the other hand, if search engines find the pages have significant differences, they may ignore the tag and crawl and index both.
Cross-Domain Canonicalization
If you are publishing the same content on two or more websites that you own, you can use the canonical tag cross-domain, much like you would use it for pages on a single website. However, keep in mind that this will also prevent the non-canonical web pages from ranking and transfer all the ranking power to the canonical URL.
Canonical Tags Aren’t the Same as Redirects
301 redirects are intended to take the user from one page to another as soon as they click the link. In essence, the user never even lands on the page they are being redirected from. However, search engines still see both pages.
Conversely, a canonical tag directs crawlers to a URL that you want crawled and indexed, however, users can still visit and interact with both pages.
Knowing how to use both and what they are intended for can help you achieve the result you are looking for.