Under the NYSE circuit breaker rules, if the value had dropped by 10 percent, or 1,100 points, at any time before 2 p.m. Friday, an automatic trading halt of one hour would have taken place, according to a NYSE description of how the breaks would work.
If the Dow were to drop by that amount between 2 p.m. and 2: 30 p.m., trading would be halted for 30 minutes. There would be no trading halt if the drop occurred later than that.
A 20 percent decline would mean a two-hour trading halt if it occurred before 1 p.m., a one-hour halt if it took place between 1 p.m. and 2 p.m. and a market closure if it happened after that. A 30 percent decline, which for this quarter would be a 3,350-point drop, would trigger an automatic halt to all trading for the day regardless of when it occurred.
The current trigger levels were set in April 1998.
The only time the triggers were pulled was in October 1997, when the Dow dropped by 350 points shortly after 2:30 p.m. That drop led to a 30-minute trading halt, but the market was later closed after the Dow continued its plunge and dropped another 550 points shortly after trading resumed that day.
That plunge and a brief one the next day were caused by economic turmoil in Asia and concerns over how it would affect the earnings of US companies according to an analysis by the Division of Market Regulation of the US Securities and Exchange Commission.