Why is the support (companion) tranche the riskiest part of a CMO, and who would buy it?

The support tranche (also called the companion tranche) is the shock absorber of a CMO structure. It absorbs all prepayment variability so that PAC and TAC tranches can maintain their scheduled cash flows. This makes it the riskiest tranche, but it also offers the highest yield.

How the Support Tranche Works:

The support tranche receives:

• Excess principal when prepayments are faster than the PAC band → its life shortens dramatically
• Deferred principal when prepayments are slower than the PAC band → its life extends dramatically

Average Life Variability — The Key Risk:

The support tranche's WAL ranges from 1.4 to 24.3 years depending on prepayments — a swing of over 20 years. This makes duration hedging nearly impossible.

Why Support Tranches Exist:

1. Mathematical necessity: You cannot create PAC/TAC protection without someone absorbing the variability. The support tranche is the residual.
2. Cross-subsidization: The tight spreads on PAC tranches (bought by banks and insurance companies) subsidize the wider spreads on support tranches.
3. Total deal economics: Without the support tranche, the CMO cannot be structured, and all investors lose access to the other tranches.

Who Buys Support Tranches?

1. Hedge funds: Active traders who can dynamically hedge prepayment risk and profit from mispriced volatility
2. Mortgage REITs: Leveraged investors seeking high current income who actively manage prepayment models
3. Proprietary trading desks: Banks trading for their own account with sophisticated hedging capabilities
4. Opportunistic investors: During market stress, support tranches can trade at deep discounts offering 15-20% yields

The Yield Compensation:

Support tranches typically yield 100-300 bps more than equivalent-maturity PAC tranches. In stressed markets, the spread can widen to 500+ bps.

CFA Exam Application: Know that the support tranche has the most average life variability and the highest prepayment risk. If a vignette asks which tranche is most affected by changes in prepayment assumptions, the answer is always the support tranche.

Practice CMO tranche ordering in our CFA fixed income question bank.