Q: What is Google Supplemental Index?

A: Google Supplemental Index is basically a scrap heap for pages in your site that don't measure up to google's standards of quality. There are basically two measures here - originality - have you just copied another page? and content do you just have a few space lines of stuff or is there real meaty content?

 You can see how many pages of your site have been indexed by google by entering something like site:www.yoursite.com and how many are consigned to the scrap heap by typing site:www.yoursite.com *** -sljktf

Too much rubbish and Google will lower the quality value it assigns to your site. Even if your site is all original and fresh look out for Google  finding summaies and pdf version of pages which it will automatically earmark as duplicates lowing your sites originalty.