The canonical premium tool makes it possible to check multiple URLs. If a canonical is included in the source code, the tool will display the URL to this canonical. Also, the canonical URL is checked for HTTP header status. A canonical must always refer to a working URL. The URL that is included as canonical is the URL that Google has to index. This must be a URL with a status 200 OK. Google also recommends using absolute URLs in the canonical element. If a relative URL is used, then this tool gives a warning: the implementation is not completely correct.