A CDS contract involves the transfer of the credit risk of municipal bonds, emerging market bonds, mortgage-backed securities, or corporate debt between two parties. It is similar to insurance because it provides the buyer of the contract, who often owns the underlying credit, with protection against default, a credit rating downgrade, or another negative "credit event." The seller of the contract assumes the credit risk that the buyer does not wish to shoulder in exchange for a periodic protection fee similar to an insurance premium, and is obligated to pay only if a negative credit event occurs. It is important to note that the CDS contract is not actually tied to a bond, but instead references it. For this reason, the bond involved in the transaction is called the "reference obligation." A contract can reference a single credit, or multiple credits.