A bond rating performs the isolated function of credit risk evaluation. There are three major rating agencies for municipal bonds: Moody's Investors Service, S&P Global (formerly Standard & Poor's) and Fitch Ratings. “AAA” - Issuers or issues rated AAA demonstrate the strongest creditworthiness relative to other US municipal or tax-exempt issuers or issues. “Aa” - Issuers or issues rated Aa demonstrate very strong creditworthiness relative to other US municipal or tax-exempt issuers or issues. Bonds in the Aa, A, and Baa are also assigned "1", "2" or "3" based on the strength of the issue within each category. Accordingly, "A1" would be the strongest group of A securities and "A3" would be the weakest A securities. S&P's ratings are similar except that all letters are capital letter and ratings within a rating category are assigned a "+" for the strongest credits within a group or "-" for the weakest credits within a group.