Bond auction tail: calculate the signal, then test its meaning
An auction tail measures the gap between a bond auction’s stop-out yield and the comparable when-issued yield. It is a useful observation, not a stand-alone verdict on sovereign demand.
Positive tail
Stop-out yield exceeds the comparable when-issued yield: the auction clears at a higher yield.
Stop-through
Stop-out yield is below the when-issued yield: the auction clears through the reference level.
Zero tail
The two yields coincide, subject to measurement precision and reference-market timing.
Bond auction tail calculator
Enter the auction stop-out yield and the when-issued yield for the same security and comparable timestamp. A yield difference of 0.01 percentage points equals one basis point.
Interpretation: what a tail can and cannot tell you
Positive tails are commonly described as weaker-than-expected clearing relative to the when-issued market, while a stop-through can indicate stronger pricing at the auction. Neither observation is sufficient to determine the full quality of demand.
| Example | Stop-out | When-issued | Tail |
|---|---|---|---|
| Illustrative positive tail | 4.275% | 4.250% | +2.5 bp |
| Illustrative stop-through | 4.225% | 4.250% | −2.5 bp |
| At reference level | 4.250% | 4.250% | 0 bp |
Compare yields for the same maturity and instrument, using a consistent when-issued reference time. Auction formats, pricing conventions and data availability differ across sovereign issuers. Auction size is contextual here and does not enter the tail formula.
Four questions beyond the headline tail
- Bid-to-cover: Was demand large relative to the amount awarded, and how does that compare with similar auctions?
- Bidder composition: Did the distribution among direct, indirect and dealer bidders change, where such categories are published?
- Market context: Did the reference yield move materially around the auction window?
- Repeatability: Is this an isolated result or part of a pattern across comparable auctions?
Explore the existing BondStats auction coverage
Read Bond Auction Tail Explained, explore the Sovereign Auction Results Database or examine a failed government bond auction scenario. This page focuses specifically on the arithmetic and interpretation of the tail rather than duplicating those resources.