What's a canonical tag, and when do I need one?
What the tag actually does
A canonical tag is a line of HTML in the page head that points to the preferred URL. It helps consolidate signals when the same content appears under several addresses, such as with or without a trailing slash. Search engines treat it as a strong hint rather than a strict command.
When you need one
Online shops with color or size filters often create many similar URLs, and each one can dilute ranking signals. Sites that serve the same article from multiple paths also benefit. A single-page brochure site with clean URLs may not need much attention here.
Keep an eye on pages that pull in traffic from several URLs, since those are the most likely to need a canonical.
- Filtered or sorted product listing pages
- URLs with tracking parameters attached
- Printer-friendly or print versions of pages
- HTTP and HTTPS or www and non-www duplicates
Setting it up correctly
Each canonical should point to a live URL that returns a normal page, not a redirect or an error. Every page should usually reference itself as canonical when it is the main version. Check the tags after any migration, because a template change can break them across the whole site.
Common mistakes
- Pointing canonicals at pages that are blocked from indexing.
- Using canonicals as a substitute for fixing real duplicate content.
