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Stewart, The canonicalization tag identifies the URL where the one true version of content can be found.  The tag used on the one true version AND all on the copies of the true version are exactly same tag because the copies will point to the true version and the true version will point to itself.  Often the version you're going to set as the one true version is that which has the shortest URL or the one without any parameters added to the end of it. The whole purpose of the canonicalization tag is to be able to say "Hey Google, I know I have a bunch of URLs that have the same content--I know it's not ideal but there's nothing I can do about that. So what I want you to do is only count this specific URL in your index so all the others don't count against me as duplicate content." Here's more detail on the topic: Canonicalization and the Canonical Tag - Learn SEO - Moz