What it means
A lender gathers similar loans into a pool, transfers them to a trust and sells certificates representing fractional ownership of that pool. Whatever borrowers pay each month, minus servicing and any guarantee fee, is passed to certificate holders in proportion to their holding.
The structure matters because it turns illiquid loans into tradeable securities, which lets lenders recycle capital into new lending instead of holding every loan to maturity. That mechanism is a large part of why long-term fixed-rate mortgages are widely available in some markets.
The cash flow pattern is unusual and catches new investors out. Each monthly payment mixes interest with a return of principal, so the balance of the investment shrinks over time and the money received must be reinvested, often at whatever rate prevails.
The dominant risk is prepayment. Borrowers can repay early, especially when rates fall and they refinance, so investors get their capital back precisely when reinvestment opportunities are poor, a pattern known as negative convexity.
Pass-throughs differ from more engineered structures. In a simple pass-through every holder shares the same cash flows pro rata, whereas a collateralised mortgage obligation carves the pool into tranches with different maturities and risk profiles, which suits investors who need a particular timing of cash flows.
Credit risk depends heavily on who stands behind the pool. Certificates issued through a government-linked agency carry a guarantee against borrower default, so the buyer is mainly taking interest rate and prepayment risk, while private-label pools leave the investor exposed to defaults as well and therefore pay a wider yield.
In practice
Real-world examples.
Example
An insurance company buys $30,000,000 of agency mortgage pass-throughs to match long-dated liabilities. It accepts monthly principal returns as the price of the extra yield over government bonds.
Example
A regional bank sells a $400,000,000 pool of its own mortgages into a pass-through structure. The loans leave its balance sheet, freeing capital to write new lending in the same year.
Example
A bond fund manager sees mortgage rates drop 1.5 percentage points and models a wave of refinancing. He expects prepayments to accelerate sharply and shortens the fund's average life assumption accordingly.
Think of it
“Pass-through passes mortgage payments directly to investors-simple distribution structure.
Formula
Calculation
Pass-through rate = weighted average coupon on the pool - servicing and guarantee fees. Investor cash flow = ownership share x (pool interest + pool principal repayments).
A trust holds a $500,000,000 pool of mortgages with a weighted average coupon of 6.00%. Servicing costs 0.35% and the guarantee fee is 0.15%, so the pass-through rate is 6.00% - 0.35% - 0.15% = 5.50%.
Across the whole pool that means borrowers pay $500,000,000 x 6.00% = $30,000,000 of interest a year, of which $500,000,000 x 0.50% = $2,500,000 is kept as fees and $27,500,000 reaches investors.
An investor buys $250,000 of certificates, which is $250,000 / $500,000,000 = 0.05% of the pool. Her monthly interest is $250,000 x 5.50% / 12 = $1,145.83. If the pool repays 1% of its principal that month, she also receives $250,000 x 0.01 = $2,500 of principal, making a total cheque of $1,145.83 + $2,500 = $3,645.83 and leaving her balance at $247,500.
Because the balance has shrunk, the next month's interest falls to $247,500 x 5.50% / 12 = $1,134.38 even though nothing about the loans has changed. That declining income stream, not a fixed coupon, is the defining feature of a pass-through.Case study
Seen in the real world.
The following is an illustrative and fictional example. Selwyn Mutual, an invented regional insurer, put $60,000,000 into mortgage pass-through certificates yielding 5.5% because the equivalent government bond paid 4.2%. Its investment committee modelled the position as if it were a ten-year bond paying a fixed coupon with the capital returned at the end.
Rates then fell by nearly two percentage points and a refinancing wave swept through the pool. In this fictional scenario, roughly $22,000,000 of principal came back within eighteen months, and the only comparable investments available paid about 3.6%, so Selwyn's income from that money dropped by around $418,000 a year.
Selwyn's illustrative correction was to model prepayment speeds explicitly, split future purchases between pass-throughs and structures with more predictable timing, and stop presenting mortgage certificates in board papers as though they had a fixed maturity date.
Watch out
Common mistakes.
- Treating a pass-through like an ordinary bond with a fixed maturity, when principal comes back gradually and unpredictably.
- Ignoring prepayment risk in a falling rate environment, which is exactly when capital is returned at the worst possible moment.
- Assuming a government-linked guarantee covers market losses, when it typically covers borrower default only and says nothing about price.
Questions
People also ask.
Why do investors receive principal every month?
Because homeowners repay part of their loan with each instalment, and a pass-through simply forwards those payments to certificate holders.
What is the difference between a pass-through and a collateralised mortgage obligation?
A pass-through shares identical cash flows pro rata, whereas a collateralised mortgage obligation slices the same pool into tranches with different maturities and risk.
How is the yield on a pass-through quoted?
Usually as a yield based on an assumed prepayment speed, so two quotes are only comparable if they use the same assumption.
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