The zero lower bound (ZLB) is the practical floor on the federal funds rate. When the Fed exhausts conventional rate cuts, it cannot push rates materially below zero without triggering deposit flight and credit tightening. That constraint forces policymakers toward unconventional tools and reshapes the entire yield curve.
The ZLB became a binding constraint during two sustained episodes:
Technically, policy rates can go negative. The ECB held its deposit rate at -0.50% from 2014 to 2022. The SNB reached -0.75%. The BOJ maintained -0.10% from 2016 to 2024. The Fed has declined to follow because of disintermediation risk. If the Fed charges banks to hold reserves, banks face pressure to reduce deposit balances or raise fees on retail customers, which tightens credit conditions rather than easing them. Cash earns exactly zero, providing a costless exit from bank deposits that prevents arbitrarily negative rates from clearing in the market. The Fed has consistently judged that the credit-tightening side effects outweigh any incremental stimulus below zero.
With the policy rate at zero, the Fed shifts to three tools that operate on longer-term rates directly.
Quantitative easing purchases long-duration assets at scale, compressing term premiums and pushing down yields across the curve. The Fed purchased approximately 3.5 trillion across QE1, QE2, and QE3 (November 2008 through October 2014). Starting in March 2020, the Fed added approximately 4.6 trillion more, running purchases at 120 billion per month before tapering began in late 2021. The full mechanics are covered on the quantitative easing page.
Forward guidance works by anchoring the expected path of future short rates, which directly pulls down long rates today. The Woodford (2003) framework formalizes this: because long rates are the average of expected future short rates plus a term premium, credible guidance that rates will stay low for years lowers the entire rate path. The Fed used "exceptionally low levels for an extended period" beginning in 2009, and "at least through 2023" following the March 2020 cuts.
Operation Twist (2011 to 2012) extended the maturity profile of the Fed's portfolio without expanding its size. The Fed sold 400 billion in short-term Treasury securities and purchased 400 billion in longer-term Treasuries, extending the average duration of its holdings by approximately 3 years. Unlike QE, this put no additional bank reserves into the system. The goal was to reduce long-term borrowing costs while leaving the overall balance sheet unchanged.
The ZLB pins the front end of the yield curve near zero while leaving the long end free to reflect growth and inflation expectations. This produces a structurally steep curve during ZLB episodes, often called the "ZLB steepener." Carry and roll-down are generous because the 2s10s spread can reach 250 bps or more. Portfolio managers willing to extend duration collect that carry against a front end that policy has immobilized.
The transition out of the ZLB tends to be abrupt. When the Fed lifts off, short rates rise faster than long rates as the market prices a rapid tightening cycle, compressing the spread. This bear flattener is the characteristic signal of the liftoff phase. The 2022 experience followed this pattern almost exactly: the 2s10s spread went from roughly +150 bps at the start of the year to an inversion exceeding -100 bps by late 2022 as the Fed hiked aggressively. The Fed funds rate chart shows the full history of both ZLB episodes and the subsequent liftoffs. Cross-reference quantitative easing to see how balance sheet expansion layered on top of rate policy during each episode.
The blog post "Ten Treasury Curve Snapshots That Tell the Story of a Generation" documents curve shapes during both ZLB episodes.