U.S. Treasury securities carry negligible credit risk backed by the full faith, credit, and taxing power of the federal government. Treasury interest is federally taxable but completely exempt from state and local income taxes. TIPS adjust their par value with CPI inflation, causing annual phantom income taxation in taxable accounts.
Government National Mortgage Association (GNMA) pass-through certificates are the only agency securities backed by the explicit full faith and credit of the U.S. government. GNMA securities face prepayment risk when interest rates fall and homeowners refinance.
Municipal securities are issued by state and local governments. General Obligation bonds require voter approval and are backed by general taxing authority. Revenue bonds fund self-supporting revenue projects (bridges, toll roads, utilities). Tax-Equivalent Yield (TEY = Municipal Yield / (1 - Tax Bracket)) calculates comparative corporate yield.