Which U.S. stock index fell about 20.5% on October 19, 1987?

The S&P 500 fell about 20.5 percent on October 19, 1987.

That day, known as Black Monday, saw a worldwide equity-market collapse. The S&P 500, a broad index of large U.S. companies, suffered a historic one-session loss. The Dow Jones Industrial Average fell even more on a percentage basis, declining 22.6 percent.

The crash reflected a combination of financial and market-structure pressures. U.S. interest rates had risen, trade tensions were prominent, and shares were considered expensive. Portfolio-insurance programs and computer-assisted selling could accelerate declines when prices moved against investors.

The S&P 500 is sometimes mistaken for a list of exactly 500 companies. It generally contains 500 leading U.S. companies, but the index is weighted by market capitalization and its membership and count can change. The Nasdaq Composite and Russell 2000 measure different groups of securities.