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:
Self-referencing Canonical Tags Are Best Practice But Not Critical
It’s best practice to have self-referencing canonical tags as the canonical tag is one of the signals that Google uses for selecting the primary page in a group of detected duplicates, however, it is not essential.
Google Will Use Other Canonicalization Factors If the Canonical Is Noindex
Google would receive conflicting signals if a canonical points to a noindex page. John suggested that Google would rely on other canonicalization factors in this scenario to decide which page should be indexed, such as internal links.
Soft 404s Cannot Be Passed to Other Pages via Redirects or Canonicals
A soft 404 cannot be passed on to another page via a canonical or redirect. Google ignores the content on a page if it detects that it is a 404 or soft 404.
Use Hreflang & Canonical Tags to Handle Partially Translated Websites
John recommends using canonical tags and hreflang tags to tell Google which language version is preferred if you only have parts of a website that are translated, instead of the entire site.
Canonical Tag May Be Ignored If Pages Aren’t Equivalent
Google will ignore canonical tags where the target page and canonicalized page are different. Canonicalization is meant to be used for identical or equivalent pages only.
Internally Linked Parameter URLs Might be Indexed Even if Canonical Points to Clean Version
When using parameter URLs for internal linking, it isn’t always guaranteed that the clean version of the URL be indexed if there is a canonical tag to this version. This is because Google has to weigh up which URL should be shown in search.
There’s No Recommended Size for ‘View All’ Pages to Be Chosen as Canonical
A ‘view all’ page doesn’t have to have a specific number of products or be a certain size in order for it to be accepted as the canonical for other pages.
Passing Signals Within a Large Group of Canonicalized Pages Takes Time
If a large group of pages is included within a canonicalized set, Google will still have to crawl and process all of these pages to find the canonical and pass on any link equity if the canonicalised pages are linked to externally.
Canonicalizing Paginated Pages Back to Main Page Can Cause Crawling & Indexing Issues
Canonicalizing pages in a paginated set back to the first page can be problematic because Google may see that these are different pages and ignore the rel canonical. Alternatively, if Google does follow the rel canonical to the main page, this could mean links and content on other pages might be missed.
Hreflang is a Minor Canonicalization Signal
Google uses hreflang as a small signal when picking the canonical version of a page, but this needs to be backed up by consistent signals provided by rel canonicals, internal linking and sitemap files.