Major U.S. market indexes fell 7% almost right after the opening bell on Monday, but despite the Dow, the S&P 500, and the Nasdaq trying to pare back some of those losses throughout the day, all three closed down just as much.

The NYSE then triggered a market circuit breaker to halt a further plunge. Circuit breakers essentially pause trading after certain decline thresholds are reached:

-If the S&P 500 falls 7%, trading will pause for 15 minutes

-If the S&P 500 falls 13%, trading will again stop for 15 minutes if stocks drop on or before 3:25 EST

-If the S&P 500 falls 20%, trading will stop for the rest of the day

They are also designed to give investors a chance to understand what’s actually happening and make trading decisions based broader market conditions.

Some of the biggest losers today were oil giants like Occidental Petroleum (OXY Quick QuoteOXY - Free Report) , Tesla (TSLA Quick QuoteTSLA - Free Report) , Dow Inc. (DOW Quick QuoteDOW - Free Report) , JPMorgan Chase (JPM Quick QuoteJPM - Free Report) , and Boeing (BA Quick QuoteBA - Free Report) .

Heading into the start of the week, investors knew there was a possibility a halt could occur. Stock futures plummeted after Saudi Arabia slashed oil prices for April, retaliating against Russia’s unwillingness to cut output. Investors were also dealing with the spread of the coronavirus, and the effect on the U.S. and global economy, as well as the decline in the benchmark 10-year Treasury note.

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