A mortgage-backed security (MBS) is a security whose payments depend on cashflows from a pool of mortgage loans. The mortgages may finance residential or commercial property. An investor buys a claim on those cashflows rather than lending directly to each property owner. The practical complication is that principal can come back at a different speed from the one assumed when the security was priced.
MBS are a form of securitisation. In a simple pass-through, investors share the pool's available principal and interest proportionately; servicing and guarantee fees reduce the cash available to them. A multiclass structure instead directs payments among tranches according to its contractual priorities. Pool cashflow and a particular investor's cashflow are therefore separate objects. The Philadelphia Fed's guide explains the pass-through mechanism.
Follow one payment date
Consider a fictional mortgage pool with USD100 million of outstanding principal just before a payment date. Borrowers repay USD5 million of scheduled principal and make an additional USD5 million of prepayments. Assume no defaults, new loans or other principal changes on this date.
The principal ledger, in USD millions, is:
One stipulated payment-date ledger in USD millions. The intermediate balance is an accounting step; actual collection and investor remittance dates depend on the security. Interest is unspecified and separate.
The USD10 million returned is repayment of principal, not USD10 million of profit. Interest is another cashflow. Measuring the investor's total return also requires the purchase price and the value of the remaining claim, as well as the relevant cash receipts and costs. A shrinking principal balance is not, by itself, evidence of a credit loss.
Nor does this example establish a standard prepayment rate. It specifies two amounts on one date. Forecasting a sequence of payments requires an amortisation schedule and explicit prepayment assumptions. The mortgage-loan and prepayment lesson develops scheduled principal and prepayment calculations (full lesson requires membership; a public preview is available).
Principal can arrive too early—or stay invested longer
For many U.S. fixed-rate residential mortgages, borrowers can repay early. Refinancing incentives often increase when mortgage rates fall, but moving house, transaction costs, eligibility and the pool's history also affect repayment. Commercial mortgages and other jurisdictions can have different prepayment restrictions.
Contraction risk arises when principal returns faster than expected. The investor loses future interest on the repaid balance and may have to reinvest at lower rates. Faster repayment can be especially costly when the investor paid a premium above principal value.
Extension risk arises when principal returns more slowly than expected. Rising mortgage rates can reduce refinancing, leaving money invested at the existing coupon for longer while market rates are higher. The investment's duration can increase as its price falls. These are conditional mechanisms, not a rule that every borrower refinances after a rate cut or that every MBS tranche responds alike. Fabozzi develops the distinction in Chapter 11, pp. 242–243.
This borrower option helps explain negative convexity in many residential pass-throughs: a rate rally can accelerate repayment and limit price appreciation. Cashflow timing must be modelled alongside discount rates.
Ask who promises which payment
A mortgage borrower can default even when an investor benefits from a separate guarantee. The guarantee changes who bears the shortfall; it does not fix the security's market price, liquidity or prepayment schedule.
For the U.S. market, the checked issuer and regulator disclosures distinguish:
| Security or guarantor | What the distinction means |
|---|---|
| Ginnie Mae-guaranteed MBS | The timely-payment guarantee has the full faith and credit backing of the United States. Ginnie Mae's security guarantee is distinct from insurance or guarantees on the underlying loans. |
| Fannie Mae MBS | Fannie Mae undertakes to supplement trust receipts for timely principal and interest payments. Its certificates are not guaranteed by the United States. |
| Freddie Mac UMBS and MBS covered by its 1 May 2026 offering circular | Freddie Mac guarantees timely interest and scheduled principal, and full and final principal by the Payment Date in the month of its contractual Final Payment Date. These obligations are Freddie Mac's, not a United States guarantee. |
| Private-label MBS | Credit protection depends on the particular deal: collateral, subordination, reserves and any contractual guarantees. Do not infer agency backing from the MBS label. |
These distinctions are documented by the SEC, Fannie Mae's programme guide, Fannie Mae's guarantee disclosure and Freddie Mac's offering circular, “Guarantee” and “Guarantees” sections, checked 27 October 2026. Payment terms vary by product; an older Freddie Mac certificate need not have the same guarantee as these UMBS and MBS.
For analysis, separate the mortgage pool's scheduled payments, voluntary prepayments and default-related receipts. Then apply servicing, fees, guarantees and payment priorities to obtain the investor's cashflows. Stress both repayment timing and credit risk, and revalue the resulting claims under different rate scenarios. A promise to pay principal and interest does not make those claims immune to market losses.
Sources
- Frank J. Fabozzi, Bond Markets, Analysis, and Strategies, ninth edition, Pearson, ©2016, Chapter 11, pp. 223–227 and 242–243: pool claims, pass-through fees and timing, contraction and extension risk.
- Federal Reserve Bank of Philadelphia: A Guide to Understanding Mortgage-Backed Securities, 18 June 2025.
- SEC Investor.gov: Mortgage-Backed Securities and Collateralized Mortgage Obligations.
- Fannie Mae: General Information About Fannie Mae's MBS Program, guide dated 6 October 2021, and Single-Family MBS guarantee disclosure.
- Freddie Mac: UMBS and MBS Offering Circular, 1 May 2026, “Guarantee” and “Guarantees” sections.