What it means
A canonical URL is declared with a <link rel="canonical" href="..."> tag in a webpage's HTML head. The tag tells search engines which URL to treat as the authoritative version of the content, even if the same content is reachable through other URLs (e.g. with tracking parameters, http vs https, or trailing slashes).
When the page IS the original, the canonical points to itself. When duplicates exist, the canonicals on the duplicates point to the original.
Why it matters
Without canonicals, duplicate or near-duplicate pages compete with each other for rankings. Each URL splits the link equity, the indexing budget, and the topical relevance signals. Setting clear canonicals consolidates everything onto one URL.
For e-commerce, canonicals matter even more because filtered category pages, sorted product lists, and tracking parameters can create dozens of duplicate URLs for the same content.
How it's used
Best practices:
- Self-reference the canonical on every page (the URL points to itself)
- Use absolute URLs, not relative ones
- Match the canonical exactly to the URL Google should index (https vs http, www vs no-www, trailing slash or not)
- Avoid pointing canonicals across very different content
- Make sure canonicals are not blocked in robots.txt and not noindexed
Canonicals are advisory, not directives. Google can ignore a canonical it disagrees with. But in most cases, a clear canonical is respected.