On the New York Stock Exchange, a rule restricting the trade of S&P 500 Index stocks when the Dow Jones Industrial Average index has gained or lost more than 2% from the previous trading day. This is done to prevent wild swings in index prices that might result from speculative investing. Rule 80A exists to attempt to maintain stability in share prices. It is also informally called the Uptick/Downtick Rule and the Collar Rule.
See Rule 80A.