After the stock market crashes of October 1987 and 1989, the New York Stock Exchange instituted a rule -- called Rule 80b -- that provided for a stop to trading should there be a rapid and significant drop in the market.
In the event of an 1,100-point decline in the Dow Jones industrial average before 2 p.m., there would be a one-hour trading halt. If the drop happens between 2 p.m. and 2:30 p.m., there would be a 30-minute halt, and after 2:30 p.m., there would be no halt in trading.
If there was a 2,200-point decline before 1 p.m., there would be a two-hour halt in trading. Between 1 p.m. and 2 p.m., there would be a one-hour halt. And if such a drop happened after 2 p.m., the market would close for the day. The market normally closes at 4 p.m.
In the event of a 3,300-point decline, the market would close for the day regardless of time.
Rule 80b has only been used once -- Oct. 27, 1997. On that day the Dow was down 350 points at 2:35 p.m. and 550 points at 3:30 p.m., shutting the market for the remainder of the day. The trigger points were much lower at that time because the value of the Dow was much lower.
With reports from ABC News' Charles Herman, Daniel Arnall and The Associated Press.