I remember the cold February when the canonical tag was announced. We were all surprised – not just because Google always told us that they could figure out duplicate content without our help, but because they were working on this in conjunction with other search engines.
The canonical tag is tag that goes into the <head> section of a page, and when implemented correctly, tells engines which original (or canonical) page should be given credit as the source:
<link rel="canonical" href="http://www.greenlaneseo.com"/>
Have several pages that are variants of one? Throw a canonical tag on all those pages, and point them to the original. Canonical tags are super helpful in ecommerce, news sites, and other dynamically created (database driven) websites.
Some algorithms run independent of the main core algorithms. Like the original Panda and Penguin algorithms, they’re not run all the time. To my knowledge it hasn’t been confirmed either way, but I believe this canonical algorithm to be a less infrequently run algorithm. I’ve seen large sites take months to have the canonical tag take hold, despite the pages being crawled several times over.
A Few More Notes About The Canonical Tag
Engines consider the URL you put into the canonical tag to be a “suggestion,” of which engines could disagree. I’ve seen some canonical tags never take effect, to the dismay of the owners of the website. I’ve also seen canonical tags with an (accidental) irrelevant URL in them get honored and mess up rankings. It’s not a perfect science, but you should be careful.
Canonical tags can also be used across domains. If a site is duplicating your content, ask them to put a canonical tag on the page.
Google treats canonical tags like a 301 redirect, so they pass PageRank.