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:
Canonicals on Shared Content to the Original Source Consolidate Authority
Canonical on Shared Content Should consolidate backlink authority on the original.
Canonical Tags Are Processed After Indexing
Canonicalised pages will be crawled, indexed, and then the canonical will be processed.
Don’t Use Noindex on Canonicalised Pages
Don’t use a Noindex on canonicalised pages, because if Google accidentally picks this as the canonical, nothing will show up in search results.
Allow Googlebot to see AB Test
If running an AB test, make sure to treat Googlebot in the same way. If you use multiple URLs, ensure that both versions are crawlable but canonicalise to the main version.
Prevent Test Site Being Indexed with Canonical
John recommends using a canonical to the main site, although he says that it’s possible for both to be indexed.
Solve Duplication with Redirects, Canonical and Linking
John recommends using redirects, canonical tags, and consistent internal linking to the primary page to solve duplication. He says Google are against using robots.txt to prevent content duplication, because Google can’t recognise the pages are duplicated if they cannot crawl it.
Use JavaScript Redirects for URLs with Fragments
In order to redirect fragment URLs, which can’t work with normal 301 redirects, you need to use JavaScript redirects, or canonical tags on the pages.
Use Canonical Tags to Resolve Trailing Slashes
Canonical tags are the best way to deal with trailng slash duplicate pages.
Many to One Canonical Tags May Be Ignored
Google will try to follow canonicals by default, but they ignore canonical tags if there are significant content differences, or if a lot of URLs canonicalising to the same page, if they think it’s a mistake.
Don’t Use Noindex on Canonicalised Pages
Don’t mix Noindex with canonical to a different page.