Level 1 Halt
A 1,200-point drop in the Dow industrial average before 2 p.m. ET will halt trading for one hour; for 30 minutes if between 2 p.m. and 2:30 p.m. ET; and have no effect if happens at 2:30 p.m. or later, unless there is a level 2 halt.
Level 2 Halt
A 2,400-point drop in the Dow industrials before 1 p.m. will halt trading for two hours; for one hour if occurs between 1 p.m. and 2 p.m.; and for the remainder of the day if at 2 p.m. or later.
Level 3 Halt
A 3,650-point drop will halt trading for the remainder of the day regardless of when the decline occurs.
The percentage levels were first implemented in April 1998, and the point levels are adjusted on the first trading day of each quarter. In 2011, those dates are Jan. 3, April 1, July 1 and Oct. 3.