The New York Stock Exchange and Nasdaq OMX Group Inc. are planning an agreement to back up each other’s closing auctions in the event of an outage similar to the nearly four-hour trading halt at NYSE earlier this month.

The closing auction is the most important part of the day for mutual funds and many institutional investors because it sets important benchmark prices for stocks and is a good time to pull off big orders without having a huge effect on the prices of shares.

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