A canonical URL is specified using the <link rel="canonical" href="..."> tag in a page’s <head>. It signals to search engines which URL should be treated as the authoritative version when the same or very similar content is accessible at multiple addresses.
Common Scenarios Requiring Canonicals
- HTTP vs HTTPS versions of a page
- Trailing slash vs non-trailing slash URLs
- URL parameters (e.g.
/product?color=bluevs/product) - Paginated series where the first page content overlaps with later pages
- Syndicated content published on multiple domains
Self-Referencing Canonicals
It is best practice for every indexable page to include a self-referencing canonical — a canonical tag pointing to its own URL. This prevents accidental canonicalisation from scrapers or syndication.
Canonical vs Noindex vs 301 Redirect
- Canonical — preserves the duplicate URL but consolidates ranking signals to the preferred version. Use when you want to keep both URLs accessible.
- Noindex — removes the page from the index entirely.
- 301 redirect — permanently redirects one URL to another. The strongest signal; use this when the duplicate URL should never be directly accessed.