Google puts websites and pages it doesn’t trust, the supplemental index exists there. The regular and continue index includes the search results that you see usually. The supplemental index results will display if there are not much documents to match the query in the main index.
If your pages or website exists in the supplemental index then it means that Google does not trust your website. Following are the different ways to get into the supplemental index:
• Not enough links
• So many low quality links
• So many low quality outbound links
• So many pages for your page rank (low PR, many pages)
• Duplicate content
• Your site is new and has a low link profile. To avoid this problem, get more authoritative links.
You can’t know if your website is in the supplemental index. If your website is new then by default, Google may place you in the supplemental index until your website gains some authority. And Google is not likely to crawl all your URLs, if your site is in the supplemental index. You should not have broken links and 404s.
Many users are still confused that the supplemental index is bad for their site or not. But all expert SEOs call the supplemental index as the sign of death which means in terms of site ranking in Google.
Now! Let’s understand what is Supplemental Result?
Supplemental Result can be defined as a URL residing in Google’s supplemental index and a secondary database include pages of less importance as measured primarily by Google’s Page Rank algorithm.
At the bottom of a search result, Google used to place a “Supplemental Result” label to indicate that it is in the supplemental index but in July 2007 they did now continue the same practice and it is no longer possible to indicate that a result is in the main one or the supplemental index.
So, I hope now you know about the supplemental index.