A rel=canonical META tag link in a webpage hints to search engines including Yahoo!, Bing, and Google--about the preferred version of the webpage to index among duplicate pages on the web.
The rel=canonical link consolidates indexing properties from the duplicates, like their inbound links, as well as specifies which URL you'd like displayed in search results.
However, rel=canonical can be a bit tricky because it's not very obvious when there's a misconfiguration--Google Webmaster Centeral has listed 5 common mistakes and the best practises to follow with rel=canonical.
Below are the five mistakes:
Google says, that the first mistake that webmaster make is adding rel=canonical to the first page of a paginated series of articles.
"Specifying a rel=canonical from page 2 (or any later page) to page 1 is not correct use of rel=canonical, as these are not duplicate pages. And, using it in this instance would result in the content on pages 2 and beyond not being indexed at all," explains Google.
In this instance, Google recommend either a rel=canonical from component pages to a single-page version of the article, or to use rel="prev" and rel="next" pagination markup.
Absolute URLs mistakenly written as relative URLs. "Relative URLs include a path "relative" to the current page. For example, "images/cupcake.png" means, while absolute URLs specify the full path--including the scheme like http://."
In these cases, Google says its algorithms may ignore the specified rel=canonical--and whatever you had hoped to accomplish with this rel=canonical will not come to fruition.
Unintended or multiple declarations of rel=canonical
Unintentional rel=canonical happens --when a webmaster copies a page template without thinking to change the target of the rel=canonical. As a result, the site owner's pages begin to specify a rel=canonical to the template author's site.
Multiple rel=canonical links to different URLs happens most frequently in conjunction with SEO plugins that often insert a default rel=canonical link. In this case, Google bots ignores all the rel=canonical hints.
Category or landing page adds rel=canonical to a featured article
In this instance, Google will not include that category page in the search results. Because, the rel=canonical in this instance signals that you would prefer search engines to display the canonical URL.
However, to be able to find both the category page and featured article, it's best to only have a self-referential rel=canonical on the category page only.
rel=canonical in the
<body></body> when encountered is disregarded. As, it's only to appear in the
<head></head> of an HTML document as early as possible.
Best practices that Google recommend for using rel=canonical:
- "Verify that most of the main text content of a duplicate page also appears in the canonical page.
- Verify the rel=canonical target doesn't contain a noindex robots meta tag
- Check that rel=canonical is only specified once (if at all) and in the
<head></head>of the page.
- Check that rel=canonical points to an existent URL with good content (i.e., not a 404, or worse, a soft 404).
- Avoid specifying rel=canonical from landing or category pages to featured articles as that will make the featured article the preferred URL in search results.
- Specify no more than one rel=canonical for a page.
- A large portion of the duplicate page's content should be present on the canonical version," Google adds.
Update: Google Panda update rolls out this Friday or Monday, according to Google's Distinguished Engineer Matt Cutts.
Also, Cutts has revealed that Google is working on a significant change to the Penguin algorithm. To paraphrase, Cutts the algorithm filter has "iterated" to date but there will be a "next generation" coming that will have a major impact on SERPs.
Also, in a new video, Cutts talks why your new page's google search ranking may drop over the time.
"When we create a new landing page with quality content, Google ranks that page on the top 30-50 for targeted keywords. Then why does the rank get decreased for the next 2 to 3 weeks? If pages didn't have required quality, then why did it get ranked in the first week?"
Cutts explains using the analogy of an earthquake said that sometimes, "Google has a hard time figuring out the original source of a new piece of content. But over time: days, weeks, months, Google is better able to figure out the most relevant result for a query due to indexing more signals over time. Thus, over time, the search results may settle down to a particular state; but early on, new pages may rank high and lose their rankings over time."