Stocks plummeted this morning as worries about the growing global coronavirus pandemic and an oil price race to the bottom weighed on global financial markets.
The New York Stock Exchange halted stock trading for 15 minutes after the S&P 500 fell 7%.
The selloff had begun in overnight futures trading, which was halted after futures contracts dropped nearly 5%.
The next circuit breaker would be if the market falls by 13%. That would pause trading for another 15 minutes.
If the market plunged 20%, everyone would go home: Trading would stop for the day.
All this comes after stocks managed to end the last turbulent week in the green. Despite three steep selloffs, the Dow also recorded its best point-gain on record last week.