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:
Content on Canonicalised Paginated Pages Will Be Lost
Unique content and links on canonicalised paginated pages will be lost if Google accepts the canonical directive.
Update Internal Links to Canonical URLs
Update internal links to canonical URLs to give Google a clean signal about which URL to index. If the canonical is accepted, any links to canonicalised URLs will be associated with the canonical URL.
Low Proportion of Indexed Pages Points to Technical Issue
If a site has a low proportion of indexed pages, this usually points to a technical issue than a quality issue. Compare the site map index counts and index status report for differences. Try splitting up sitemap file , checking indexed pages using info: query, that rel canonicals match those in sitemap file, hreflang and internal linking. Also, uppercase, lowercase, trailing slashes all matter. Then check crawl stats to get idea of crawl rate and if it’s reasonable.
Align Linking & Rel Canonical If Want Particular Page Indexed
Ensure internal links and rel canonical are pointing to preferred page to ensure you aren’t giving Google conflicting signals about which page should be indexed.
Google Takes Rel Canonical To Be Mistake If Present With Noindex
If a canonical and noindex are both present on a page, Google’s algorithms judge the rel canonical to be a mistake and the noindex as being used as a way to force a canonical.
Add Self-Referential Canonical Tags
Add self-referential rel canonicals to pages as it gives Google a clear indication of what page is to be indexed. Even if there is just one page, there may be different variations such as parameters – a rel canonical will clean these up.
Search Console HTML Improvements Are Created Before Canonicals Are Processed
HTML Improvements in Search Console are generated when pages are initially crawled, but before a canonical tag is processed so they can be ignored for canonicalised pages.
Internal & Sitemap Links May Override Canonical Tags
Google uses a number of factors to determine which URLs to show. Canonicalised pages may still be chosen if you link to them internally and in Sitemaps.
Use Canonical Tags to Test a New Website Before Migration
When you want to test a new website in parallel with the old one, you can launch the new site on a sub-domain and canonicalise the new pages to old pages.
Content in Iframes May be Indexed on the Embedding Page
Pages embedded within an iframe on another page may be indexed as content on the embedding page as it will be seen when the page is rendered. You can use X-Frame-Options to prevent browsers from embedding a page which Google will respect.