# Canonical URL (rel=\"canonical\") ## What it is The canonical link tells crawlers which URL is the authoritative version of a page when the same content is reachable through several addresses: ```html ``` It belongs in `` and was standardised in [RFC 6596](https://www.rfc-editor.org/rfc/rfc6596). All major search engines honour it. ## Why it matters The same content often lives at multiple URLs without anyone meaning it to: - `https://example.com/page` and `https://www.example.com/page` - `http://` and `https://` versions of the same path - `/page` and `/page/` (with and without trailing slash) - `/page?utm_source=newsletter` and `/page` (tracking parameters) - `/page?sort=date` and `/page?sort=name` (filter and sort variants) - Paginated archives, printer-friendly views, AMP versions Without a canonical, crawlers may index every variant separately, splitting ranking signals and backlinks across duplicates. With a canonical, those signals consolidate on one URL. The result is a stronger ranking for the page you actually want to rank, and a cleaner index. A canonical is a **strong hint**, not an absolute directive. Search engines can override it if other signals (sitemaps, redirects, internal links) point elsewhere. Make all your signals agree. ## How to implement Put a **self-referencing canonical** on every public HTML page. Even pages without duplicates benefit: it removes ambiguity for query-string variants and short-circuits any scraper that copies the page. ```html ``` Rules: - **Use absolute URLs.** Include the scheme (`https://`) and host. Relative canonicals are allowed by the spec but more error-prone; absolute is the safer default. - **Match the scheme you actually serve.** If the site is HTTPS everywhere, canonicals must say `https://`. - **Pick one host and stick to it.** Either `example.com` or `www.example.com`, never both. Redirect the other. - **Be consistent about trailing slashes.** If `/articles/` and `/articles` resolve to the same content, pick one and canonicalise to it. Mixed is the worst case. - **Strip tracking parameters** from the canonical. The canonical of `/page?utm_source=newsletter` is `/page`. - **Avoid loops.** Page A canonical to B, B canonical to A is a contradiction. - **One per page.** Multiple canonicals are ignored. Cross-domain canonicals are valid when content is syndicated. If you republish an article on a partner site, the partner can canonical back to your original: ```html ``` For pages that exist in multiple languages, use `hreflang` in addition to `canonical`. Each language version is its own canonical; `hreflang` ties them together. The canonical can also be sent as an HTTP header, which is useful for non-HTML resources like PDFs: ```http Link: ; rel="canonical" ``` ## Common mistakes - Pointing every page's canonical at the homepage. This tells crawlers your inner pages are duplicates of the homepage and de-indexes them. - Canonicalising paginated archives (`?page=2`) to page 1. Each pagination page is distinct content; let them index normally. - Setting a canonical on `https://` that points to `http://`, or vice versa. - Forgetting to update the canonical when the site moves domain. - Generating canonicals from `window.location` in client-side JS only. Crawlers may not run JS; render server-side. ## Verification - View source on a few pages with query parameters. The canonical should ignore the parameters and point to the clean URL. - Run `document.querySelector('link[rel=canonical]').href` in DevTools. - Crawl the site and list canonical targets. Every target should return 200 OK, not a redirect. - In Google Search Console, the "URL Inspection" tool shows the user-declared canonical and the Google-selected canonical. When they differ, something else is overriding your hint.