Canonicalization
Canonicalization is a method used to help prevent duplicate content issues and manage the indexing of URLs in search engines. Using canonicals appropriately can be hugely helpful for SEO.
Implementing the canonical tag link attribute “rel=canonical” is a signal to search engines about the preferred page for indexing, and will be followed in most cases when it is correctly implemented to an equivalent page.
The collected SEO Office Hours notes below provide detailed information and best practices (straight from Google’s own search experts) for using canonicals on your website.
For more on canonical tags and related topics, check out Lumar’s additional resources:
Google Ignores Many Pages Which Canonicalise to the Homepage
Google recognises there may be an issue if many pages have a rel canonical to the homepage and will therefore likely be ignored.
Google Requires Rel Alternate to Detect AMP Pages
Google requires a rel alternate to AMP pages to detect the pages, and a rel canonical from the AMP page back to desired canonical page.
Migrate From Separate Mobile to Responsive Before Mobile First
If you are migrating your separate mobile pages, set up 301 redirects from your mobile URLs to your canonical pages. Rankings should not be affected before mobile-first indexing, however you might see more fluctuations after mobile-first indexing as your mobile URLs will be the canonical pages, and it will be seen as a site migration.
Use Info: to Find Canonical URLs for Hreflang Tags
If your hreflang tags point to non-canonical URLs, they won’t be accepted. Check the canonical for any URL by searching for it with info:
Websites are Responsible for User Generated Content Quality
If you allow users to add content to a website, you are responsible for any content quality issues. You can use solutions like canonicals and noindex.
Canonicalise Faceted pages to Non-filtered Version
Google recommends allowing crawling of faceted pages but canonicalise to non-filtered version of that page instead of blocking them with robots.txt.
Mobile-First Index will Support AMP Pages
AMP pages will be used in the mobile-first index if they are set up as separate mobile URLs, or if the AMP page is your canonical page.
Remove Incorrect Canonical Tags Until They Can be Corrected
Google has algorithms to catch common mistakes made by webmasters with canonical tag implementation, but they may not work, so John recommends to remove incorrect canonical tags until they can be implemented correctly.
AMP Requires Mobile Rel Alternate to be a Mobile Page
If you want to use an AMP pages as your mobile version, it requires a mobile rel alternate from the desktop pages. The AMP page should already contain a canonical back to the desktop.
Info Query to Find Canonical URLs
Search for a URL with an Info: query to see if a different URL has been indexed instead.