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:
AJAX is not 100% equivalent to normal static HTML pages, but it should be possible to make pages rank as well. Avoid using fragments in URLs, and check with Fetch and Render.
Google May Choose an HTTP URL as Canonical if the HTTPS has Mixed Content
Google will try and choose a canonical URL which does not have mixed content issues from http/https. I.e. if there is an https version of a page that DOES have mixed content issues, but an http version that does NOT have mixed content issues, then the http version could be used. If however, there are very strong signs to use that https version coming from the website, (such as redirects or rel=canonicals) then Google will still use the https version even though it has mixed content issues.
Don’t Mix Noindex and Canonical
Having a page with a canonical tag pointing to a page with a noindex is a problem but the canonical might just be ignored.
Canonicalising Product Variants Loses Unique Content
If you have product variant pages, instead of noindexing, you are better to canonicalise the variations to a single version which can consolidate all the ranking signals. However you will lose any unique content from those variations.
Googlebot Can Be Redirected to Canonical URLs
John says that althought it’s technically cloaking, it’s actually OK to redirect Googlebot from URLs with tracking parameters to canonical URLs, but allow users not to be redirected, so they can be tracked in analytics.
Duplicate Pages with a Noindex May Be Selected
If you have duplicate pages with the same content which aren’t canonicalised, but one of the pages has a noindex, then Google might pick the noindex version, and then that page will be noindexed even if there is an indexable duplicate.
Internal links should use canonical URLs
Avoid internal linking to non-canonical URLs.
Noindexing paginated pages is OK
But canonicalsing makes much more sense which he doesn’t mention.
Only use indexable canonical URLs for app deep links
Links on paginated pages
In response to a question about whether it is ok to canonicalize back to page one of a paginated set, John expanded more on whether Google counts links within pagination: