If the U.S. Treasury market is facing a multitude of difficulties, one options trading strategy whose popularity has skyrocketed is among the least discussed phenomena. This strategy is commonly known as a Box Spread: it combines four options at two strike prices, a bull call spread and a bear put spread (mostly referencing the S&P 500 Index SPX), to construct a market-neutral position. When the position is opened, its price is the cash received by the seller (the borrower of funds); the fixed payout amount at expiration is the total funds ultimately recovered by the buyer (the lender of funds). The difference between the two is the "interest" paid to the lender of funds. It is arguably the closest instrument to a risk-free return in the derivatives market, and it has another major advantage: the product is an equity derivative, so returns are taxed as capital gains, at a lower rate than ordinary interest income. According to data from Cboe Global Markets, last Friday, the outstanding loan scale corresponding to S&P 500 options box spreads hit a record high of $146 billion. Last month, the average daily notional trading volume exceeded $2.3 billion, up 26% year-over-year; among that, average daily trading volume from retail traders reached $54 million. Currently, a 3-month S&P box spread can achieve a yield of more than 4.4%; by comparison, the 3-month U.S. Treasury yield is below 4%, and the overnight secured financing rate SOFR is about 3.9%. Henry Schwartz, Vice President of Derivatives Market Intelligence, said in an email: "ETF issuers, wealth advisors, and even some large retail accounts are increasingly using this tool. Whether lending or borrowing funds, its effective interest rate is better than other available alternatives." Box spreads were once regarded as an ingenious but complex strategy, available only to high-end institutional clients and large banks. Now, as ETFs package the strategy into low-cost products, ordinary investors and financial advisors are gradually becoming familiar with options-based strategies, and this trade is moving into the mass market. At present, at least three ETFs track this strategy, with combined assets under management of about $20 billion; the largest is Alpha Architect's $15 billion 1-3 month BOXX ETF. Although its scale is still tiny compared with the $30 trillion U.S. Treasury market, the rapid growth of box spreads reflects that financial advisors and individual investors are actively seeking alternatives to traditional fixed-income assets and embracing so-called "tax-optimized" investment strategies. This also objectively puts pressure on the Federal Reserve: the Fed needs to push interest rates higher to compete with the yield levels of other assets. But the strategy also has hidden risks: the larger the trading volume, the more attention it will attract from government regulators. In July, the U.S. Treasury Department said it was closely monitoring various investment strategies intended to circumvent U.S. tax regulations. Box spread funds, as well as other tax-reduction strategies used by financial advisors, have come under regulatory scrutiny. So far, regulators have not issued new rules targeting this trade. But some bond market observers worry whether the strategy's rising popularity could divert funds that would otherwise have gone into the bond market. A financial advisor who requested anonymity (while waiting for the government to clarify its stance on box spread ETFs) said: "Box spreads have existed for a long time; it's just that now overall options trading volume is rising, and liquidity is no longer a bottleneck. Investors can use various derivative variants, and in the past, these funds would mostly have been allocated to fixed income or cash-like assets."