The S&P 500 triggered the first circuit-breaker trading halt on 9 March 2020.
U.S. stock trading paused after the S&P 500 fell 7% from its previous close shortly after the market opened. This was a Level 1 market-wide circuit breaker, designed to give participants time to assess rapidly changing conditions.
The halt occurred during the 2020 stock-market crash, when investors responded to the spread of COVID-19, economic shutdowns, and an oil-price shock caused by a breakdown in cooperation between major oil producers. Trading halts also occurred on 12 March, 16 March, and 18 March.
The mechanism had been introduced after the 1987 crash and revised over time. It is based on percentage moves in the S&P 500, although the halt applies across U.S. equity markets rather than only to companies in that index.