What was the 1987 market-crash mechanism that automatically paused U.S. stock trading called?

The 1987 market-crash mechanism that automatically paused U.S. stock trading was called circuit breakers.

Circuit breakers are temporary trading halts triggered when a market or security moves beyond specified thresholds. They were developed after the Black Monday crash of October 19, 1987, when the Dow Jones Industrial Average lost 22.6 percent in one session. Regulators wanted a pause that could interrupt panic selling and allow information to circulate.

The New York Stock Exchange introduced market-wide circuit breakers in 1988. Modern U.S. rules use percentage thresholds tied to the S&P 500: Level 1 and Level 2 halts can last 15 minutes, while a Level 3 decline stops trading for the rest of the session.

Circuit breakers do not prevent prices from falling and are different from margin calls, which require investors with borrowed money to provide additional funds or sell assets. Their purpose is to slow trading, not guarantee a price.