After stock market crashes in the late 1980s, the Big Board implemented circuit breakers to force traders to take a break from frenzied selling. The triggers were updated in 1998. In 1997, the market was shut down under the previous circuit-breaker triggers.

If the Dow Jones industrial average falls 1,100 points �?equivalent to about 10 percent at the beginning of the quarter �?before 2 p.m., the market will shut down for an hour. If the threshold is breached between 2 p.m. and 2:30 p.m., the halt will last 30 minutes. Trading will continue to take place if stocks plummet 1,100 points after 2:30 p.m.

If the index falls 2,200 points �?which at the beginning of this quarter was about 20 percent �?before 1 p.m., the market will close for two hours. If the decline takes place between 1 p.m. and 2 p.m., there will be a one-hour pause. The market will close for the day if stocks sink 2,200 points after 2 p.m.

In the event of a 3,350-point decline, the market would close for the day, regardless of the time.

The thresholds are computed at the beginning of each quarter to establish a specific point value for the quarter. The 1,100-point drop represents a 10 percent decline; the 2,200 level, a 20 percent drop, and the 3,350 level is a 30 percent drop.