Collateralized Mortgage Obligation
A collateralized mortgage obligation (CMO) is a type of mortgage-backed security that pools residential mortgage loans or agency MBS pass-throughs and redirects their principal and interest cash flows into multiple tranches (bond classes) with different maturities, prepayment sensitivities, and risk profiles.
Key takeaways
- CMOs were created to redistribute prepayment risk inherent in mortgage pass-throughs, allowing different investor groups to select their preferred exposure to interest rate and prepayment uncertainty.
- Sequential pay tranches direct all principal payments to the first (shortest) tranche until it is retired, then to subsequent tranches — creating graduated maturity profiles from a single pool.
- PAC (Planned Amortization Class) tranches use a prepayment 'collar' (range of assumed prepayment speeds) to provide highly predictable principal payment schedules — the most stable CMO tranche type.
- IO (Interest Only) and PO (Principal Only) strips separate the interest and principal components of mortgage cash flows, creating instruments with dramatically different interest rate sensitivities.
- CMOs exhibit negative convexity because prepayments accelerate when rates fall (limiting price appreciation) and slow when rates rise (extending duration and deepening losses).
- Agency CMOs are backed by Ginnie Mae, Fannie Mae, or Freddie Mac guarantees; non-agency (private label) CMOs rely on internal credit enhancement and are directly exposed to mortgage credit risk.
Explanation
CMOs were invented in 1983 by Salomon Brothers and First Boston to solve a fundamental problem with mortgage pass-throughs: their unpredictable cash flows made them unsuitable for many institutional investors with specific liability matching or maturity requirements. By tranching the underlying mortgage pool's cash flows, CMO structures created differentiated instruments that could match the specific needs of pension funds (long, stable cash flows), money market funds (short, predictable durations), and banks (intermediate maturities).
The standard sequential pay CMO structure divides the pool into tranches A, B, C, and Z. All principal payments from the mortgage pool flow first to Tranche A until it is fully retired. Only then do principal payments flow to Tranche B, and so on. Tranche Z (the 'accrual' or Z-bond) receives no cash flows until all prior tranches are retired, accumulating accrued interest that is added to its outstanding balance. This sequential structure creates short, intermediate, and long-duration instruments from the same mortgage pool.
Prepayment risk is the central analytical challenge in CMO investing. Mortgage borrowers have an embedded call option: they can prepay (refinance) their mortgages when interest rates fall below their mortgage rate. This creates negative convexity — when rates fall, prepayments increase, shortening the CMO's duration and capping price appreciation. When rates rise, prepayments slow (extension risk), extending duration precisely when higher-duration assets are losing the most value. The PSA (Public Securities Association) prepayment model standardizes prepayment speed assumptions, with 100 PSA being the baseline assumption.
PAC (Planned Amortization Class) tranches address prepayment uncertainty by defining a prepayment collar (e.g., 100-300 PSA). Within this range, PAC tranches receive exactly scheduled principal payments regardless of actual prepayment speeds, with 'companion' or 'support' tranches absorbing the variability. PAC investors sacrifice yield relative to companion investors in exchange for stable, predictable cash flows. When prepayments fall outside the collar, PAC schedules also become uncertain — but within the collar, PACs provide near-bond-like cash flow predictability.
IO and PO strips represent the most interest-rate-sensitive CMO structures. An IO strip receives only the interest portion of mortgage payments — its cash flow is zero when the underlying mortgage is fully prepaid. As rates fall and prepayments accelerate, the IO's remaining cash flows are dramatically reduced, causing IO prices to decline when rates fall (unusual behavior — most bonds rise when rates fall). PO strips receive only principal, appreciating dramatically when rates fall and prepayments accelerate (large principal return shortly rather than over 30 years). These properties make IOs and POs powerful, if complex, hedging instruments for mortgage-heavy portfolios.
Formula
CMO Tranche Duration ≈ Weighted Average Life × Price Sensitivity Factor; PAC schedule determined by min/max principal payment at upper and lower PSA collar speeds
Example
A $1 billion CMO is structured in four sequential pay tranches: Tranche A ($300M, average life 3 years), Tranche B ($250M, average life 7 years), Tranche C ($250M, average life 12 years), and Z-bond ($200M, 20+ year average life). The underlying pool consists of $1 billion in 6.5% 30-year fixed rate mortgages. At a 150 PSA prepayment speed, Tranche A's average life is 3.2 years with a duration of 2.8 years — suitable for an insurance company liability matching a group annuity. If prepayment speeds increase to 400 PSA (a refinancing wave), Tranche A's average life shortens to 1.8 years and must reinvest principal at lower prevailing rates — the manifestation of prepayment/reinvestment risk for the Tranche A investor.
Related terms
Accrued Interest Annuity Bankers Acceptance Bond Call Option Collar Convexity Duration Exchange Extension Risk Federal Funds Rate Hedging