Many companies and governments that issue bonds pay agencies such as Standard & Poor's, Moody's Investor Service or Fitch Ratings to rate their bonds. These ratings are to bond issuers what FICO scores are to consumers. They reflect how likely the borrowers are to make their full principal and interest payments.

The highest rating is AAA. As companies and investors have become more blasé about debt, the number of AAA-rated corporate bonds has fallen. In 1979, Moody's rated 61 companies AAA. Today, that number is less than 10.

Bond issuers can raise their rating - sometimes to AAA - buy purchasing bond insurance, which promises to repay bond investors if the issuer can't.

To see each agency's rating scale, go to http://www.bondsonline.com/asp/research/bondratings.asp