# Canonical URL

> A canonical URL is the preferred address of a page, set with `<link rel='canonical'>`. It tells search engines which version to index when the same content has several URLs.

- **Check ID:** S5
- **Area:** [SEO](https://be1st.ai/glossary/seo)
- **Tool:** [SEO + Security + Performance](https://be1st.ai/tools/ssp-audit)
- **Page:** https://be1st.ai/glossary/seo/canonical-url

## Why it matters

Without a canonical tag, search engines may index multiple versions of the same page (with/without www, with/without trailing slash, with UTM parameters), which splits link equity and can lower rankings.

## In practice

Shopify automatically adds canonical tags to all product pages so that URLs with filters (`?color=red`) do not take ranking away from the main product page. Without this, Google could index hundreds of duplicates.

## How be1st checks it

Checks for the presence of a canonical tag (`<link rel='canonical'>`), which specifies the preferred version of a page. It helps prevent duplicate content issues.

[SEO + Security + Performance](https://be1st.ai/tools/ssp-audit), SEO tab.

Every check ends as: Pass, Warning, Fail.

## Sources

- [Consolidate Duplicate URLs](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls) (Google Search Central)
- [HTML `<link>` Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/link) (MDN Web Docs)

## Related terms

- [Trailing slash consistency](https://be1st.ai/glossary/seo/trailing-slash-consistency): Trailing slash consistency means every URL is used in one form only, either /page or /page/, and the other form redirects to it.
- [Viewport meta tag](https://be1st.ai/glossary/seo/viewport-meta-tag): The viewport meta tag tells mobile browsers how wide to lay out a page and how to scale it. Without it, a phone shows a shrunken desktop page.
- [Robots meta tag](https://be1st.ai/glossary/seo/robots-meta-tag): The robots meta tag gives search engines instructions for one page, such as noindex (keep it out of results) or nofollow (do not follow its links).
- [Open Graph tags](https://be1st.ai/glossary/seo/open-graph-tags): Open Graph tags are meta tags with the og: prefix that set the title, description and image a page shows when it is shared on Facebook, LinkedIn, Slack and other apps.
- [Twitter Card tags](https://be1st.ai/glossary/seo/twitter-card-tags): Twitter Card tags are meta tags with the twitter: prefix that control how a link preview looks when a page is shared on X, formerly Twitter.
- [Structured data](https://be1st.ai/glossary/seo/structured-data): Structured data is machine-readable markup, usually JSON-LD with the Schema.org vocabulary, that says what a page contains: a product, an article, a company or an event.