method of evaluating the possibility of default by a bond issuer. Fitch Ratings, Standard & Poor's, and Moody's Investors Service analyze the financial strength of each bond's issuer, whether a corporation or a government body. Their ratings range from AAA (highly unlikely to default) to D (in default). Bonds rated BB or below are not investment grade-in other words, institutions that invest other people's money may not under most state laws buy them.