Federal regulators in April approved steeper limits to the point drops that halt trading. The new rules impose 10 percent, 20 percent, and 30 percent limits, and are recalculated quarterly.
A 900 point drop before 2 p.m. would stop trading for an hour, and the halt would last 30 minutes if the drop occurred between 2 p.m. and 2:30 p.m. There would be no halt if the 10-percent drop occurred in the last 90 minutes of trading.
The NYSE would halt trading for an hour if the index lost 1,750 points (about 20 percent) before 2 p.m. Trading would be halted for the rest of the day if the Dow lost 1,750 points during the last two hours of trading.
A 2,650-point loss at any time would halt trading for the remainder of the day.
Long-standing limits on computer-run program trading kick in when the market swings 50 points in either direction.
Previous rules suspended trading for a half-hour if the Dow dropped 350 points and for one hour if the index fell 550 points.
The market would close for the day if the 350 level was reached at 3:30 p.m. or if the 550 level was hit at 3 p.m. under the old rules. That last occurred Oct. 27, 1997, when the Dow fell 554.25 points, about 7 percent.