A Residential Mortgage Backed Security (RMBS) is a type of security that is backed by a pool of mortgages on residential properties. These securities are typically issued by a government-sponsored enterprise (GSE) such as Fannie Mae or Freddie Mac, or by a private issuer. The mortgages in the pool are used as collateral for the security, and the cash flows from the mortgages are used to pay investors. RMBS are typically sold in tranches, with each tranche having different levels of risk and return.