LONDON (MarketWatch) -- The New York Stock Exchange imposes trading halts in the event of extreme market volatility.

The so-called circuit breaker rules first went into effect following the 1987 market crash.

The rules call for trading halts of differing lengths in the event of declines of 10%, 20%, and 30% in the Dow Jones Industrial Average DJIA, -0.47% based on the average closing value of the average over the month immediately prior to the start of the current quarter.

For the current quarter the rules call for a 60-minute halt in trading in the event of a 1,350 point decline in the Dow Jones Industrial Average reached before 2 p.m.; a 30-minute halt in the event of a 1,350 point decline reached between 2 p.m. and 2:30 p.m., and no halt in trading in the event of a 1,350 decline reached after 2:30 p.m.

The timing of the halts changes in the event of a larger decline.

The rules call for a 120-minute halt in trading in the event of a 2,700 point decline in the Dow Jones Industrial Average reached before 1 p.m.; a 60-minute halt in the event of a 2,700 point decline reached between 1 p.m. and 2 p.m.; and market closure in the event of a 2,700 point decline reached after 2 p.m.

A decline of 4,000 points in the Dow industrials at any point during the day would trigger closure of the market for the remainder of the day.