Avoiding Google Supplemental Index
Google is believed to have stored web pages in two separate indexes: Main index and supplemental index. Supplemental results are part of Google’s auxiliary index and pages, which appear on supplemental listing. They also have fewer restrictions than those are drawn from the main index. This is Google’s explanation on supplemental results:
A supplemental result is just like a regular web result, except that it’s pulled from our supplemental index. We’re able to place fewer restraints on sites that we crawl for this supplemental index than we do on sites that are crawled for our main index. For example, the number of parameters in a URL might exclude a site from being crawled for inclusion in our main index; however, it could still be crawled and added to our supplemental index.
If you’re a webmaster, please note that the index in which a site is included is completely automated; there’s no way to select or change the index in which a site appears. Please also be assured that the index in which a site is included doesn’t affect its PageRank.
Web pages in Google’s main index are the prominent pages and will almost always show up first in any search query. Web pages in the supplemental index will only show up if there are very few or no results available within the main index. If your web pages are stored in Google’s supplemental results, then they will not be returned regularly in search engine result pages.
The types of web pages that are very likely to be included in the supplemental index have the symptoms below:
* Web pages with no text content or little text content
* Web pages that have text content duplicated across multiple different pages
* Web pages with many repetitive keywords in title tag or meta description tag
* Web pages that have very long URLs or URLs with many dashes
* Web pages with no inbound link or inbound links pointing from very low quality pages
[...] I recommend reading one of my previous posts if you intent to avoid your web pages from getting into the Google Supplemental Index. [...]
[...] However for most sites, all of the above URLs usually return the same page. Therefore what Google sees is identical content on different URLs. If you are fortunate enough not to have run into duplicate content issues, Google picks one of the URLs and stores the others in the supplemental index. [...]