As the yield curve flattened to the lowest in more than a decade, the fallout spread beyond the realms of high finance and central banking.

It also caused the value of hundreds of millions of dollars worth of debt -- often held by retail investors -- to evaporate.

Holders of ‘‘steepener’’ securities are facing the prospect of minuscule or even zero coupons. The structured products were issued in droves in recent years by Wall Street banks including Citigroup Inc., Goldman Sachs Group Inc., JPMorgan Chase & Co. and Morgan Stanley. Frequently marketed by brokers, they pay a high introductory fixed rate that switches to a floating coupon linked to the gap between short- and long-term U.S. interest rates.

The complex math behind the trade has been laid bare by the flattening curve, leaving a slew of retail investors blindsided, say critics, as issuance booms.

Globally, around $2.5 billion of notes tied to constant maturity swap rates was sold last quarter -- the most since 2015 -- scores of which use the steepener structure, according to data compiled by Bloomberg.

Spokespersons at the banks declined to comment on the impact of the yield curve on the notes.

While banks issue the debt securities, they’re often marketed and sold to investors via intermediaries such as brokerages. In some cases, the latter have been censured and fined by the Financial Industry Regulatory Authority for failing to ensure the products were suitable for their clients. No banks were named in these actions.

“They were sold as being conservative investments, which of course they’re not,” said Jeffrey Pederson, an attorney in Denver, Colorado, who has represented investors in settlements against financial institutions over whether such notes were suitable for his clients.

Pederson said the buyers in question are prevented from discussing the cases thanks to non-disclosure agreements.

The products -- to a large degree, a bullish bet on U.S. growth and inflation over the long haul -- share the same basic structure. The steeper the slope, the higher the coupon, up to a cap. If the curve levels out -- as it has been over the past year -- or inverts, buyers can be stuck with measly coupons or even no interest payments.

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