Use of the Canonical Tag, Both Internally and Cross Domain

[GearyLSF37]

I've seen the cross domain canonical not work at all in my test cases. And an interesting point was brought to my attention today. That point was that in order for the canonical tag to work, the page that you are referencing needs to have the exact same content. And that this was the whole point of the canonical tag, not for it to be used as a 301 but for it to consolidate pages with the same content.

I want to know if this is true.

Does the page you reference with a canonical tag have to have the same exact content?

And what have been your experiences with using the canonical tag referencing another page on a different domain that has the same exact subject matter but not the exact duplicate content?

[RyanKent]

Canonicalization is indeed designed to prevent duplicate URLs to the same content. It is designed to funnel users to the correct version of the page, and also to ensure your page is properly ranked.

In some cases, it doesn't make any difference from a user point of view. Whether you type in www.mysite.com/cars or www.mysite.com/cars/ (note the ending slash added) a user may be taken to the same page (or not). But if you have www.mysite.com/cars?sort=asc&field=color then that is probably not the page you want users landing on. You can canonicalize all the various sort ordered pages along with the various URL variations of that same page into one.

That is what canonicalization is designed to accomplish,not merge pages with similar but different content.

[JaspalX]

Rand mentioned using cross-site canonicals at the linklove event in London, perhaps some of the SEOMoz staff have some data on that kind of usage that they could share? (particularly with respect to the how closely content needs to match from page to page).