Stock trading was briefly halted shortly after the market opened on Monday after the S&P 500 declined sharply, triggering an automatic 15-minute “circuit-breaker” that kicks in when the index falls by 7 percent.

The 7 percent threshold was passed about four minutes after the opening bell Monday, halting trading until 9:49 am.

Shortly after stocks resumed trading, the S&P, Dow Jones Industrial Average, and the Nasdaq Composite were all down by around 6 percent, indicating a small bounce for stocks from the level that halted trading.

Futures on the big equities index hit trading curbs on Sunday night and early Monday morning when they fell below 5 percent of Friday’s close.

Stock trading circuit-breakers are thresholds that trigger trading halts when stocks fall below certain levels.

A Level 1 halt, triggered four-minutes after the open Monday, occurs when the S&P 500 drops by 7 percent before 3:25 p.m. Trading is halted for 15 minutes.

A Level 2 halt, which halts trading for an additional 15 minutes, is triggered if stocks fall by an additional 6 percent, a 13 percent overall decline, from the prior close before 3:25 p.m.

A 20 percent decline triggers a Level 3 halt, which stops trading for the remainder of the day.