The exchange, a unit of Intercontinental Exchange, still needs to make technological changes before the new rules can go into effect. Kristen Kaus, a spokeswoman for the NYSE, would not provide any detail about how long that might take.
The rules themselves are somewhat technical. Unlike other exchanges, which are nearly fully automated, at the NYSE people on the trading floor open the stocks, a process the exchange says gives it greater stability because people can intervene in ways that trading algorithms cannot.
For an individual stock to begin trading on a typical day, the trader responsible for that stock - known as the market maker - has to disclose the likely price at which a stock will open, known as pre-opening indications.
On volatile days or when trading was disordered, the exchange could invoke its "Rule 48," allowing market makers to open trading on stocks manually on a case-by-case basis, even if they could not identify a stable price. Because it required human judgment and interaction, Rule 48 sometimes slowed the ability of shares to start or resume trading.
The newly approved procedures would eliminate Rule 48 and instead set up specific guidelines for when shares could open or be reopened after a trading halt. The theory is that this will allow trading to start more quickly.
"This is really a consolidation to make it a bit more deterministic, orderly and more rules-based, so it is a very good thing, smart move by the exchange," said Jamie Selway, a market structure analyst at ITG in New York.
ETFs and Rule 48
The new procedures would always allow individual stocks to open without pre-market indicators if they were set to trade within 5 percent of their previous day's close. On particularly volatile days, those shares could open as long as their prices were within 10 percent of their previous close.
The exchange could declare a volatile day and go to the 10-percent guide under any one of three scenarios: if futures contracts on the Standard & Poor's 500 index, called e-mini futures ESc1, are pointing to an open more than 2 percent away from the closing price the prior day; if the exchanges are attempting to reopen after a market-wide trading half; if the exchanges determine it is necessary to maintain a fair and orderly market.
Members of the exchange-traded fund community have been critical of Rule 48 and other attempts by exchanges to curb market volatility, citing the extreme market volatility suffered last Aug. 24.
On that day, trading halts caused particular problems for ETFs because they are required to trade their underlying securities when investors trade the ETFs themselves. But shares of underlying securities were halted while the ETFs themselves were not, causing ETF prices to fall much more sharply than declines in their net asset values would have caused.
Dave Nadig, director of exchange traded funds at FactSet Research Systems Inc, said the rule change was a step in the right direction. "While those aren't going to solve all of our problems in what are increasingly fragile equity markets they are certainly going to go a long way," he said.
But not all market participants believe the new rules will be any help in the event of the next big sell off.
Market participants will "really realize that it's a mistake when one day the markets are really under stress and stocks just open like that and there is no opportunity to slow it down," said Ken Polcari, director of the NYSE floor division at O'Neil Securities in New York.