A type of debt security backed by a group (or pool) of fixed income assets ranging from mortgage-backed securities to corporate bonds sold in the market to credit protection sold by the issuer under credit default swaps (CDS) in a synthetic CDO. To maximize the range of investors to whom they can be sold, the securities are divided into different classes. These typically consist of senior and subordinated tranches (often including an equity tranche at the most junior level) that have differing priorities regarding payment of principal and interest, and offer differing maturities and interest rates. The more senior tranches have the right to priority of payment over more junior tranches, but the more junior tranches carry a higher rate of interest, as they are riskier assets. Each tranche is generally rated by a credit rating agency.