With stocks plunging, here are the circuit breaker rules

• The S&P 500 fell 20% on Oct. 19, 1987, the single largest one-day decline in its history; yesterday it closed down 4.4%.
• Since 1987, the NYSE instituted market-wide circuit breakers that it can use to halt trading "if a severe market price decline reaches levels that may exhaust market liquidity."
• The NYSE can halt trading for 15 minutes when the S&P falls 7% from the prior day's closing price; then another 15 minutes if it's down 13%.
• If the S&P 500 falls 20% from the prior session's close, it's halted for the rest of the day.
• This is why it's important to not focus so much on the point drop, but on the percentage decline.
• In 1987, the Dow Jones Industrial Average fell 508 points, a 22% plunge at the time. Yesterday, the Dow fell 1,190.95 points, a 4.4% drop.