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I noticed that StackOverflow uses a rel=canonical tag on page 2+ of answers. For example, see http://stackoverflow.com/questions/6470651/creating-a-memory-leak-with-java?page=2&tab=votes#tab-top

Notice they have a rel=canonical tag that points to http://stackoverflow.com/questions/6470651/creating-a-memory-leak-with-java

As I recall, Maile Ohye at SMX Advanced said that you should NOT use a canonical tag in this way. The answers on page 2 are not duplicate content as to what's on page one, and duplicate content is the purpose of the canonical tag.

Is this an appopriate use of the rel=canonical tag?

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That's a use of canonicalization, and arguably the most important one, but from the Webmaster Tools content guidelines, pagination:

You can also add a rel="canonical" link to the component pages to tell Google that the View All version is the version you want to appear in search results.

Even more specific to your example above, the use case is that the rel-canonical information also makes an allowance for differences in sort. The SO question you linked to isn't only paginated, but also sorted. Selecting a different tab puts an entirely different set of answers onto "page 2" that could continue moving even in the future depending upon votes, etc.

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Sorting shouldn't enter into it - the canonical tag ought to simply link to the current page number without any sorting. –  DisgruntledGoat Jan 18 '12 at 17:51
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I would consider that an abuse of rel=canonical, and I'm rather surprised that Google seems to be honoring it. (All the results I see are from mirror sites without the rel=canonical link, whereas searching for the top-voted answer works fine.)

It would be one thing if the canonical link pointed to a "view all" page, but (AFAIK) StackOverflow doesn't have those. Here, the second page contains a substantial amount of content that the supposedly canonical page lacks.

Then again, I guess the net effect of doing this isn't so different from just putting noindex meta tags on the second and subsequent pages, except that all the PageRank for those pages is shifted to the "canonical" page instead of being diffused across outgoing links. So I guess I can see why Google might consider this tolerable, even if seems like doing a disservice to the readers.

(The proper thing to do for such pages, IMO, would be to use rel=prev and rel=next on the pagination links, which SO already seems to do. According to the page Su' linked to, Google takes these attributes into account and generally leads visitors to the first page, presumably unless they specifically search for something only found on the later pages.)


Update: Google seems to agree:

"While it's fine to set rel="canonical" from a component URL to a single view-all page, setting the canonical to the first page of a parameter-less sequence is considered improper usage. We make no promises to honor this implementation of rel="canonical"."

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