Factbox: When the NYSE invokes Rule 48

(Reuters) - The New York Stock Exchange and NYSE Amex Cash Markets invoked Rule 48 on Tuesday, after a selloff in Europe raised fears of rocky trading at the open of U.S. markets.

Rule 48 is invoked to speed up and smooth trading at the market open when exceptionally high volatility is expected.

Other facts about Rule 48:

* It allows the exchange to ease back on requirements for price indications that help determine the floor price during regular sessions. Bypassing the requirement helps speed the opening of trading.

* Among triggers for invoking the rule is "substantial activity in the futures market before the open," according to the exchange's website. Other factors involve expectations of high volatility, which may be driven by news or market conditions.

* The rule may be invoked in a reopening of trade following a market-wide halt of trading at the exchange, according to the website, but NYSE Euronext spokesman Ray Pellecchia said halts only occur if the Dow Jones industrial average <.DJI> moves more than 10 percent in a day. The last time a halt occurred was in October 1997, he said, when the Dow fell more than 500 points in all. Halts at that time were based on points and not percentages, Pellecchia said.

* Rule 48 was last invoked Tuesday, and the three major indexes opened down more than 2 percent on fears the euro zone's sovereign debt crisis was worsening and the U.S. economy was slipping back into a recession.

* Rule 48 also was invoked three times last month: Aug 8, 10 and 18, as well as on March 15, January 12 and January 27, Pellecchia said.

* The latest version of the rule was adopted on December 6, 2007, according to the website.