Canonical Tag
A canonical tag is an HTML element naming the preferred URL when the same content is reachable at several different addresses. It tells search engines which version to index and rank, consolidating ranking signals instead of splitting them across near-identical duplicates.
Why it matters for your rankings
Duplicate URLs are created constantly without anyone deciding to make them. Tracking parameters, filter combinations, session IDs, print views, trailing slashes and http versus https all produce distinct URLs serving the same content.
Left alone, Google picks a canonical for you, and its choice is often wrong. Your links, engagement and authority get attributed to a URL with a tracking parameter attached, while the clean version you actually promote sits unindexed. On ecommerce sites with faceted navigation the effect is severe: a hundred products can generate tens of thousands of crawlable URLs, exhausting crawl budget on combinations nobody searches for. The canonical tag is how you tell Google which single version deserves the credit.
How to check it on your site
Check every template has one
View source on a product, category, article and homepage. Each should contain a self-referencing canonical unless it is deliberately pointing elsewhere.
Confirm URLs are absolute
Canonical values must be full URLs including protocol and domain. Relative paths are a frequent and silent failure.
Look for conflicting signals
A page should not carry a canonical to one URL while a redirect, hreflang or sitemap entry points somewhere else. Crawl with Screaming Frog and check the Canonicals tab for 'Canonicalised' pages.
Verify Google agrees
Use the URL Inspection tool in Search Console. It reports both your declared canonical and the one Google selected. Disagreement means your signal is being overruled.