# Canonical URL Checker

Identify duplicate content issues instantly. Verify if your pages are correctly using the canonical tag to protect your SEO.

## Stop keyword cannibalisation

Duplicate content splits a page against itself. Three pages of near-identical content give a search engine no way to choose, so it often ranks none of them well. This checks that bots are being pointed at the version you actually want to rank.

### When you need a canonical check
- Ecommerce. One product reachable through several categories, so `/shirts/red-shirt` and `/sale/red-shirt` are the same page twice.
- URL parameters. Campaign tags like `?utm_source=facebook` look like a new page to a crawler unless a canonical says otherwise.
- Syndicated content. If you republish on Medium or LinkedIn, your own post should be the canonical version.

### Reading the result
- Self-referencing. The page points at itself. Normal, and what a unique page should do.
- Points elsewhere. Correct for a duplicate: a paginated result, a print version, a syndicated copy.
- Missing. No instruction at all, so a search engine guesses which version to rank.

## Questions
### What is a canonical URL?
### Why is duplicate content bad for SEO?
### What does 'Self-Referencing' mean?
### Should every page have a canonical tag?
### Can I use this for non-HTML files?