Alan Greenspan was the chair of the Federal Reserve — the US central bank — for nearly 18½ years, from August 1987 to January 2006. No one has held that job longer across multiple presidents. He served under Ronald Reagan, George H.W. Bush, Bill Clinton, and George W. Bush.

To understand why this matters: Fed chairs can serve multiple terms, but each time they need the sitting president to reappoint them. The fact that Greenspan survived being reappointed through a Democratic president (Clinton) sandwiched between Republicans was a sign Congress and the White House agreed he was doing the job well — or at least doing something they could live with.

Greenspan took office in August 1987. Just weeks later, the stock market crashed on a single day in October, shedding more than one-fifth of its value. This is called the Black Monday crash. Greenspan and the Fed's response was to pump cash into the financial system and publicly promise they'd keep lending to banks if things got worse. That playbook — add cash, stay visible, be ready to act — became the template central banks use when crisis hits. Over his 18 years, Greenspan dealt with the savings and loan crisis, two recessions, a failed hedge fund, the dot-com bubble bursting, and the panic after 9/11.

Interest rates are what the Fed uses to manage the economy. When the Fed lowers its target interest rate, borrowing becomes cheaper, so people spend and invest more. When it raises rates, borrowing costs more, so people spend less and inflation slows. Under Greenspan, rates moved in a wide band. When he started, rates were above 9%. After the dot-com crash in 2000 and 9/11, he cut them down to 1% — near zero — and left them there for years.

That decision to keep rates so low for so long became controversial after the 2008 financial crisis. Many people blamed it for helping fuel the housing bubble that triggered the crash. But economists still debate how much blame actually belongs to low rates versus other causes.

Greenspan was famous for talking in a confusing way. He once said to Congress that if his meaning was ever clear, he had probably made a mistake. The idea was: if you say exactly what you'll do next, investors will bet on it and force your hand. Better to keep markets guessing. Later Fed chairs rejected this approach completely. Today the Fed publishes forecasts and holds press conferences to explain what it's doing.

When Greenspan retired in 2006, he was seen as one of the most important central bankers ever. But the 2008 financial crisis changed that story. Greenspan himself told Congress afterward that he had believed banks would police their own risk-taking for their own profit — and he was wrong.

The plain facts: 18 years, four presidents, one of the longest tenures ever. The story people take from those facts, though, depends on how they see his era. Some point to the 1990s as proof that smart, active Fed policy can sustain long growth. Others point to the years leading up to 2008 as a cautionary tale about what happens when central bankers trust their models too much and fail to see danger ahead. Both views look at the same history and come away disagreeing.