Call Rule

In the world of commodities trading (think wheat, corn, pork bellies), there's a common sense call rule that the opening bid price of a particular cash commodity has to be on or near the closing price from the previous day. The call rule came about because naughty traders continued to make off-the-book deals after the markets closed, making it unfair to others and creating pricing chaos in the marketplace.

The rule was established in 1906 by the Chicago Board of Trade, following the lead of the Commodities Futures Trading Commission. So if on July 21 the price of potash is $59 a pound at the end of the trading day, that has to be the opening price on July 22 (or whatever the next trading day is, if it falls on a weekend or holiday).