The single most important number in American finance is the federal funds rate. It sets the floor under the reader's mortgage, his credit card, and his savings account. When the news says the Fed raised or cut rates, this is the rate they mean.

Here is the strange part. The rate is named after, and calculated from, a market where banks lend each other cash overnight. But almost no banks do that anymore. The market has shrunk to a hollow shell, and the little activity left in it is mostly one narrow money-making trick between a government lender and foreign bank branches.

The machine we are drawing is that market, the federal funds market. It once set the true cost of money for the whole banking system. Today it is a near-fiction that still gives its name to the world's most-watched interest rate, and understanding the gap is understanding how the Fed really works now.

The market is a ghost of what it was. Daily volume now runs a bit over $100 billion, tiny next to the trillions moving through other funding markets, and the trades are concentrated in a handful of players. The textbook picture of banks lending each other spare reserves has all but vanished.

What is left is an arbitrage. The only real lenders now are the Federal Home Loan Banks, and the main borrowers are the U.S. branches of foreign banks. The foreign banks borrow cash cheaply and immediately redeposit it at the Fed at a slightly higher rate, pocketing the thin spread. That trade is most of the market.

Why this reaches the reader: the rate that governs his entire financial life is now produced by this small, lopsided market. The Fed no longer controls it the old way. It steers the rate with other levers entirely, while the famous market that gives the rate its name has quietly emptied out.

The board of a federal bank voted unanimously to lend nearly $3 billion to build a gold mine on American soil. Congress got 25 days notice. Nobody objected.

Final papers are expected in the second half of this year. The day that ink dries, three things happen at once: funding risk goes to zero, the U.S. government becomes financially fused to the project, and Wall Street re-rates the stock from speculative developer to federally backed strategic asset.

One more detail. The company's own filings cite "substantial support and partnership from the Department of War," a phrase we've never seen on a gold project. The reason: alongside its gold, the deposit holds a metal China formally banned from export to the United States. The only domestic reserve in the country.

The company is about one fiftieth the size of Newmont.

See the gold stock at the center of it >>

Start with how it used to work. Every bank had to keep a minimum of reserves at the Fed. On any given night some banks ended up short and some had a surplus, so the ones with extra lent to the ones who needed it, overnight and unsecured. The average rate on all that lending was the fed funds rate, and it was the banking system's true marginal cost of cash.

That made the rate a perfect policy tool. The Fed could add or drain reserves to make them scarcer or more plentiful, which moved the rate banks charged each other, which rippled out to every other interest rate. The fed funds market was the lever, and it was genuinely a market of banks trading with banks.

The load-bearing part broke after 2008, when the Fed flooded the system with reserves. Suddenly almost no bank was short; everyone had plenty. With no scarcity, there was nothing to borrow for. At the same time, new regulations made it costly for domestic banks to lend or borrow here at all. The reason the market existed, redistributing scarce reserves, simply disappeared.

So the market hollowed out, and a workaround filled the vacuum. The Fed now pays interest on the reserves banks park with it, and that rate, not interbank trading, is the real lever. The Fed sets the policy rate by administrative decree and uses the interest it pays on reserves to steer it, rather than by making reserves scarce.

What remains in the fed funds market is an arbitrage around that very tool. Foreign bank branches, which face lower regulatory costs, borrow cash from the Federal Home Loan Banks at a hair below the Fed's reserve rate, then park it at the Fed to earn that reserve rate. The whole market is now largely two types of institution exploiting a few basis points of spread, and its average rate is what the world still calls the fed funds rate.

The first joint is representativeness. The flagship rate is now set by a thin, lopsided market dominated by a few lenders and a few borrowers doing one trade. When a benchmark that steers trillions in loans is produced by a shrinking pool of niche activity, it can jump around for reasons that have nothing to do with the broader cost of money, sending a noisy signal to everything pegged to it.

The second joint is concentration. Essentially one set of lenders, the Federal Home Loan Banks, supplies the cash, and a narrow group of foreign bank branches does the borrowing. If those players pull back, say, when their own cash runs low at quarter-end, the rate can pop upward unexpectedly, as it has in recent episodes, not because policy changed but because a few arbitrageurs stepped away.

The third joint is the illusion of control. Because the Fed now sets the rate administratively, the system looks firmly in hand. But the Fed is steering a rate produced by a market it no longer truly drives, relying on its administered rates to keep everything aligned. That alignment holds in calm times and can slip when cash gets scarce, which is why policymakers are openly debating whether to replace this aging benchmark with one built on the far larger repo market.

What persists: the rate's authority, which is intact. Whatever the plumbing beneath it, the fed funds rate remains the anchor for the entire structure of American interest rates, and the Fed's administered-rate system has kept it inside the target range reliably for years. The signal still works, even though the machine producing it has changed beyond recognition.

What can break: the link between the benchmark and reality, at the margins, when the thin market misbehaves. A rate set by a shrunken, arbitrage-driven corner can give false signals or lurch at quarter-ends, and because so much is pegged to it, even small distortions ripple outward. The deeper risk is complacency, trusting a number as if it still came from a deep, representative market when it no longer does.

This machine connects to two others the reader's money touches. Its only real lenders are the Federal Home Loan Banks, the same system whose advances to member banks we have traced, and the Fed now steers this rate using the interest it pays on reserves, the same administered rates that set the floor in the reverse repo facility we covered. The benchmark, the FHLBs, and the Fed's reserve rate are three parts of one control system.

The next reading arrives in the daily fed funds volume and the ongoing debate over whether to anchor policy to the repo market instead. Those will show whether the world keeps steering by a rate drawn from a near-empty market, or whether the benchmark finally moves to where the money actually trades.