Futures on the Dow Jones Industrial average, the S&P 500 and Nasdaq-100 all hit the so-called limit down threshold at one point, off by more than 5%, before paring those losses slightly. At last count, the Dow futures were right at that threshold, which halts trading. Dow futures implied a loss of more than 1,100 points at the open. S&P 500 and Nasdaq-100 futures were off by 4.8%.

Exchanges halt trading of futures contracts if they drop by more than 5%, acting as a floor for selling until regular trading resumes at the opening bell at 9:30 a.m. ET. After stocks open in regular trading, the S&P 500 must drop by 7% before triggering the New York Stock Exchange’s circuit breaker, which halts trading temporarily.

Thursday was the second time this week that exchanges were forced to halt futures trading prior to the market open: Futures hit a similar “limit down” prior to the opening of normal trading Monday morning. Later that day, the S&P 500 also hit NYSE’s 7% circuit breaker and the Dow fell more than 2,000 points.