Which U.S. stock index triggered the first market-wide circuit breaker on October 27, 1997?

The Dow Jones Industrial Average triggered the first U.S. market-wide circuit breaker on October 27, 1997.

The Dow fell 554 points, or 7.2%, during that session, which became known as Mini-Crash or Black Monday. The decline followed major turbulence in Asian markets and renewed concerns about global financial stability. Because the fall exceeded a predefined threshold, trading was halted under rules introduced after the 1987 crash.

The circuit-breaker system was designed to give investors time to assess information and reduce the chance that automated or panic selling would feed on itself. The original rules used percentage declines in the Dow, although later systems were redesigned around levels in the S&P 500.

This event is different from the 2010 Flash Crash, which involved a much shorter intraday disruption, and from the 1987 crash itself. Modern U.S. exchanges now use tiered S&P 500 circuit breakers at specified percentage declines.