Rates · R5 · 8 min
Supply, auctions and relative value
Tell whether a government bond auction went well, explain why the newest bonds yield a little less, and say how heavy borrowing moves the curve.
In the news: “The 10-year auction tailed 2bp”: whether a sale of government debt went well, and why the rest of the market cares.
The essentials
- Auction formats
- US Treasury auctions are single-price: bids fill from the lowest up, and all winners get the highest yield accepted, the stop-out. At conventional auctions, run by the , winners pay what they bid.
- Bid-to-cover
- Total bids divided by the amount sold: $105bn of bids for $42bn of bonds is about 2.5. It means little on its own, so traders compare it with recent auctions of the same bond.
- Tail and stop-through
- The is how far the stop-out lands above the yield at the bidding deadline: 4.238% against 4.215% is a weak 2.3. Landing below it is a stop-through, a sign of strong demand.
- On-the-run bonds
- The newest bond at each maturity is : the one everyone quotes, trades and borrows. Investors pay for that ease, so it yields a few basis points less than an older bond maturing at about the same time.
- Supply and the curve
- Heavier long-dated issuance means investors must be paid more to hold it, so long yields rise against short ones and the curve steepens, even if expected don’t move.
Judging a US auction
Tail (bp) = (stop-out yield − when-issued yield) × 100
- stop-out
- the highest yield accepted at the auction, %
- when-issued
- the new bond’s yield in the market at the bidding deadline, %
How it works
1. Governments sell bonds at auction
When a government needs to borrow, it sells new bonds at an auction. The US Treasury runs its own; in the UK the does. Investors bid by saying how much they want and the yield they want for it. Asking for a higher yield is the same as offering a lower price.
2. The stop-out yield
In a US auction the Treasury fills bids starting from the lowest yield and works up until the bonds are sold. Everyone who wins gets the same yield: the highest one accepted, called the stop-out. Bids at higher yields get nothing.
3. Bid-to-cover
The is total bids divided by the amount sold. $105bn of bids for $42bn of bonds is 2.5: buyers wanted two and a half times what was on offer. On its own the number means little, so traders compare it with recent auctions of the same bond.
4. Tails and stop-throughs
Before the auction the new bond already trades, called , so the market shows what yield it expects. The is the stop-out minus that yield at the bidding deadline: 4.238% against 4.215% is a 2.3bp tail, meaning buyers needed more yield than the market showed. A negative tail is a stop-through, a sign of strong demand.
5. Reading the whole result
A strong auction stops through, has a bid-to-cover above its recent average, and leaves little with the , the banks that must bid at every auction. A weak one tails, covers poorly and leaves dealers holding a lot, which they then sell. That can push yields up across the market for the rest of the day.
6. Gilt auctions measure the tail differently
At a UK conventional gilt auction, each winner pays the price it bid rather than one common price. So the DMO measures the tail inside the auction: the yield at the lowest price it accepted minus the yield at the average price. The smaller the tail, the more tightly the bids were grouped.
7. New bonds yield a little less
The newest bond at each maturity is : everyone quotes, trades and hedges with it, so it is the easiest to buy and sell. Investors pay for that ease with a slightly lower yield. An older bond maturing at almost the same time usually yields a few basis points more.
8. More supply, steeper curve
When a government borrows more, there are more bonds to sell, and where they land matters. Lots of extra 30-year bonds means investors must be paid more to hold all that long-term risk, the . Long yields rise relative to short ones, so the curve steepens, even if nobody expects to change.
Worked example
The US Treasury sells $42bn of 10-year notes. At the bidding deadline the when-issued yield is 4.215%. Bids total $105.4bn, the auction stops out at 4.238%, and dealers are left with 18% against a recent average of 12%.
- Tail: (4.238 − 4.215) × 100 = 2.3bp.
- Bid-to-cover: 105.4 ÷ 42 = 2.51.
- Dealers took 18%, well above their usual 12%, so end investors bought less than usual.
- Read: a weak auction. Expect 10-year yields to drift higher after the result.
A 2.3bp tail, a 2.51 bid-to-cover and a heavy dealer share: a weak auction.
Check yourself
8 quick questions on the explanation above.
Practice
New numbers every set. Misses and guesses go into your review deck.
In the interview
Answer out loud in under a minute, then compare.
What makes an auction tail, and how might the market react?
Why do on-the-run bonds trade rich to older issues?
How does heavier government issuance affect the curve?
Remember
Tail = stop-out yield − when-issued yield
An auction that tails sold for less than the market price; one that stops through found more demand than was on offer.
Sources
- UK Debt Management Office — Official Operations in the Gilt Market: an Operational Notice (1 April 2026)
- UK Debt Management Office — Auction result: 4⅝% Treasury Gilt 2032, 22 September 2026 (tail definition)
Definitions and maths are standard textbook material; dated facts link to the sources above. Worked numbers are illustrative.