A type of bond having mortgages or mortgage-backed securities as collateral. Principal and interest payments from an underlying pool of mortgages are redirected to pay the CMO holders until the CMOs are retired. A single issue of CMOs contains two or more classes of bonds called tranches, each with a different length of maturity, providing a form of call protection to the holder of a CMO. A holder who wants to lock in a CMO investment for a specific length of time will buy into a tranche with a low risk of being retired early because the underlying mortgages are paid off early. Such low prepayment risk tranches are called planned amortization classes (PACs). Changes in prepayment rates in the underlying pool of mortgages are absorbed first by another tranche, so that the PAC remains unaffected by prepayment risk. CMOs generally pay principal and interest semiannually. CMO were first issued by the Federal Home Loan Mortgage Corporation (Freddie Mac) in June 1983.