The tail at a Treasury auction is the difference between the highest yield accepted at the auction (the "stop") and the yield at which the security was trading in the when-issued market just before the auction. It is reported in basis points.
Positive tail (auction stops through): the auction yield exceeds the when-issued yield, meaning the government had to pay more than the market expected. This signals weak demand.
Negative tail (auction stops rich): the auction yield is below the when-issued yield, meaning demand was stronger than expected. This is a strong result.
On the screws: the auction prices exactly at the when-issued yield.
Tail size matters relative to history. A 1 bp tail on a 2-year note auction is more significant than a 1 bp tail on a 30-year bond auction because the shorter-maturity market is tighter. The auctions tool normalizes each tenor's tail against its own historical distribution.
Tails are among the most closely watched auction metrics because they provide a real-time read on demand that isn't available from the bid-to-cover ratio alone. A high bid-to-cover can coexist with a positive tail if bidders are concentrated at lower prices.
Large positive tails (3+ bps) can trigger immediate selloffs across the curve, while negative tails often spark rallies.
Treasury Auction— The process by which the U.S. government sells new debt securities to fund operations. Auction results signal demand strength via bid-to-cover, tail, and bidder allocations.
Bid-to-Cover Ratio— The ratio of total bids to the amount sold at a Treasury auction, measuring demand strength. The single most-cited metric for gauging investor appetite at issuance.
When-Issued— Trading in a Treasury security before it has been formally issued, establishing the pre-auction benchmark yield.
Primary Dealer— A financial institution authorized to trade directly with the Federal Reserve and required to participate in Treasury auctions.