The S&P 500 traded as low as 5,101.75 on Friday, down more than 5%. While this certainly feels like a significant decline, the index would have to drop to 5,018.76 to trigger the first circuit ...

After that, a 20% decline would trigger a Level 3 circuit breaker, shutting down trading for the day. After 3:25 p.m., trading halts only in the event of a 20% fall.

The steep selloff was again threatening to put the market's circuit-breaker rule in play. That dictates that a fall of 7% for the S&P 500 before 3:25 p.m. Eastern time results in a marketwide ...

If the S&P 500 drops 20%, and triggers a level 3 circuit breaker, trading would stop for the rest of the day. The S&P 500 traded as low as 5,101.75 on Friday, down more than 5%.

Here's a breakdown of what it takes to trigger the circuit breakers: -- A Level 1 circuit breaker stops trade for a minimum of 15 minutes if the S&P 500 falls 7% before 3:25 p.m. Eastern time.