Which major U.S. exchange closed for four days after the September 11 attacks, reopening on September 17, 2001?

The New York Stock Exchange closed for four days after the September 11 attacks and reopened on September 17, 2001.

The attacks destroyed the World Trade Center complex in Lower Manhattan, near the NYSE and other financial institutions. U.S. securities markets did not open on the scheduled trading days from September 11 through September 14 because of security, communications, transportation, and operational concerns.

Trading resumed on Monday, September 17. The Dow Jones Industrial Average fell 684.81 points, or 7.1 percent, that day, making it the largest one-day point decline at the time. Markets remained volatile in the following sessions.

The NYSE is often confused with NASDAQ when discussing the closure because both are major U.S. markets. The NYSE is the specific exchange named in the question; the broader U.S. market shutdown affected multiple trading venues.