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 Will Usually Drop Session IDs from URLs
Instead of choosing a representative URL for a set of URLs with session IDs, Google will usually drop the session ID from the URLs completely if it recognises that they don’t return any unique content.
Google Would View a Page Canonicalized to a Noindex URL as a Noindexed Page
If you have a canonical link pointing to a page that is noindexed, the page canonicalised to it would also be considered noindex. This is because Google would view it as a redirect to a noindex page and therefore drop it.
There is No Risk of a Noindex Signal Being Transferred to the Target Canonical Page
If a page is marked as noindex and also has a canonical link to an indexable page, there is no risk of the noindex signal being transferred to the target canonical page.
Having Canonical Tags Set For Parameter URLs Helps Google Understand Connections Between Pages
Google tries to figure out the canonical URL for parameter pages that are included in the GSC parameter handling tool, so they may crawl these pages to identify and understand the canonical set up and connection.
Canonical Tags Should Remain The Same Between Desktop & Mobile Sites Even With Mobile First Indexing
If you have a separate m. version of your site, the canonical should remain pointing to the desktop version, despite mobile-first indexing. This is because Google will use the canonical tag to understand which of the pages belong together. Internally, Google will pick the mobile version as canonical.
Use Redirects & Canonical Tags to Stop Data From Other Site on Same IP Being Included in GSC
If there are internal links between two sites on the same IP address, data for both sites can sometimes appear in the same GSC account. If you don’t want Google crawling the other site then add redirects or canonical tags pointing to the main site.
Canonicalization is Determined on Per-page Basis not Sitewide
Google ignoring the canonical tags across an entire site because of incorrect implementation would be very rare and something that a Google engineer would need to do manually. Canonicalization is determined on a page-by-page basis and uses a number of other factors than just the canonical tag, including URL information and on-page content.
Google Takes Several Factors into Consideration When Selecting a Canonical When Multiple Variations Exist
If you have a large number of variations of one URL, for example, multiple parameters, Google will take into consideration a number of factors including canonical tags, internal linking and the sitemap file, before picking a canonical version.
Hreflang Not Necessary For Non-Canonical Pages
It isn’t necessary to implement hreflang on the non-canonical version of a page because it won’t be shown in search, therefore any hreflang annotation won’t be used by Google.
Canonical Tag Can be on Either AMP Page or Legacy Page within an AMP HTML Pair
If you have the rel=amphtml tag set up correctly, Google can pick up the canonical tag from either page in the pairing.