TBA Trading and Dollar Rolls

Buy a stock and you know exactly what you own. Buy an agency mortgage bond in the most liquid way possible and, at the moment you trade, you don't. TBA stands for "to-be-announced": you agree on issuer, coupon, face amount, price, and settlement month, and only two days before settlement — on notification day — does your counterparty tell you which specific pools you are actually getting. It sounds like it should be the illiquid corner of the market. It is, in fact, one of the most liquid fixed-income markets in the world, and the mechanism behind that is worth understanding on its own.

Why not just trade specific pools

If every trade required picking exact pools first, a buyer would need to underwrite thousands of individual pools for every transaction, and liquidity would collapse — exactly the illiquidity securitization was built to escape in the first place (see What Securitization Does and Why It Exists). TBA sidesteps this: the market agrees that any pool meeting good-delivery standards (same coupon, same issuer, weighted average maturity and loan age within set bands) is acceptable, so a trader can quote and hedge a coupon without caring which pools eventually show up. Specificity is deferred to just before settlement, when the seller chooses the cheapest-to-deliver pools that still qualify — a small optionality that is priced into TBA versus specified pool trading, where a buyer pays up for pools with known, favorable prepayment characteristics.

The dollar roll

A dollar roll is a trade built entirely out of TBA's structure: sell a TBA pool for near-term settlement and simultaneously buy the same coupon back for a later settlement month, at a lower price. Economically this looks like a repo — you are financing a position and will get equivalent (not identical) collateral back next month — except the price drop, the "roll," is negotiated directly rather than being an explicit repo rate.

The roll is attractive, or "special," when it implies financing cheaper than the actual repo rate available in the market, and that specialness usually shows up exactly when prepayments on that coupon are expected to be high — the seller of the roll effectively gives up this month's prepayments (since they're delivering old pools now and getting different pools back next month) and dealers who need to cover shorts are willing to pay up for the roll, driving the implied financing rate down further.

Worked example: pricing a roll as implied financing

Suppose Fannie Mae 5.5s trade at 99-16 (99.5) for front-month settlement and 99-04 (99.125) for next-month settlement — a roll of 12/32, or 0.375 points, on a $10 million position.

$$0.375\% \times 10{,}000{,}000 = 37{,}500$$

That $37,500 is the amount the seller of the roll effectively earns for giving up one month of ownership (and one month of coupon and any prepayments) on $10 million face. Convert it to an annualized financing rate: dividing by the front price and annualizing a one-month drop of 0.377 percent ($0.375/99.5$) gives roughly $0.377\% \times 12 = 4.52\%$ implied financing. If actual repo for the same collateral is trading at 5.25 percent, the roll is "special" — the buyer of the roll (the one rolling their position forward) is financing at 4.52 percent, about 73 basis points cheaper than repo, which is attractive enough that real-money holders systematically roll their positions forward instead of taking delivery and financing in repo directly.

Where you meet it in practice

TBA and the dollar roll sit underneath nearly every agency MBS trading desk's activity: mortgage originators hedge their pipeline by selling TBA forward, real-money managers hold synthetic MBS exposure by continuously rolling rather than settling, and the roll market is itself a read on prepayment expectations and repo conditions. Anyone quoting "MBS current coupon" is quoting a TBA price, not a specific bond.