And while we're still a bit away from the point at which the S&P 500 will stop trading for a period, Dave Lutz at JonesTrading circulated the "circuit breaker" levels at which the S&P 500 will be halted.

In an email on Monday, Lutz wrote:

A cross-market trading halt can be triggered at three circuit breaker thresholds on the S&P:

7% (Level 1), 13% (Level 2), and 20% (Level 3).

These triggers are set by the markets at point levels that are calculated daily based on the prior day's closing price of the S&P 500 Index.

A market decline that triggers a Level 1 or Level 2 circuit breaker before 3:25 p.m. will halt market-wide trading for 15 minutes, while a similar market decline "at or after" 3:25 p.m. will not halt market-wide trading. A market decline that triggers a Level 3 circuit breaker, at any time during the trading day, will halt market-wide trading for the remainder of the trading day.

Level 1 = ~1832
Level 2 = ~1714
Level 3 = ~1575