Opening Cross takes place when the NASDAQ market pairs off all of the buy and sell orders in an in each security listed on NASDAQ. The opening cross determines the NASDAQ official opening price.
Applying "Opening Cross" to Securities Exams:
The Opening cross begins at 9:28 AM. At this time the NASDAQ Market Center Execution System automatically executes orders. Orders placed after 9:28 AM may not be canceled. Orders placed after 9:28 AM may only be changed if the change to the order makes the order more aggressive. A change that increases the size of the order or improves the price would make the order more aggressive.