A group of mortgages held in trust as collateral for the issuance of a mortgage-backed security. Some mortgage-backed securities issued by Fannie Mae, Freddie Mac and Ginnie Mae are known as "pools" themselves. These are the simplest form of mortgage-backed security. They are also known as "pass-throughs" and trade in the to-be-announced (TBA) forward market. 

|||Pass-throughs or pools are comprised of mortgages with close to the same maturity and interest rate. However, a pool of mortgages that backs a more complex mortgage-backed security or CDO might consist of mortgages of more varying interest rates and characteristics.