Definition: A canonical URL, sometimes referred to as a "canonical tag," is an HTML attribute that tells search engines where to assign the search value, or "link juice," for pages with duplicate or very similar content. Online stores often use canonical tags for product pages, which is crucial for retaining full search value.
In its efforts to eliminate low quality content from search engine results, Google algorithms such as Panda penalized websites for having "duplicate content." Any page that(s) with the majority of content appearing on another webpage (same domain or any other) is considered duplicate. However, there are legitimate reasons to have live pages with similar content. On top of intentional actions, web servers frequently create duplicate versions of a page. The canonical tag solves for this, notifying search engines of the "canon" page to receive search engine value for a group of similar content.
Located in the <head> section of a webpage, canonical tags appear as follows:
For example, there are two URLs for different variations of a product:
Each page can use the canonical tag to avoid duplicate content:
Both 301s and canonical URLs can be used to solve for duplicate content issues. While a 301 ensures a user can no longer land on a page — they are redirected to an assigned URL — canonical URLs are accessible.
301: Old promotions, out-of-date content, and deprecated URLs can all be redirected to their closest equivalents. This ensures that any users — or search engines — trying to navigate won't get a 404 response code.
Canonical: Pages with multiple versions, such as product pages with unique URLs for every variation of item, should use the canonical URL. When search engines find each page, the canonical tag tells it to attribute full value to one version. Use the canonical tag when you want a page to remain accessible.