A complex option trading strategy (originally a box) that involves selling a bull call spread and a corresponding bear put spread with the same strike prices and expiration dates. Essentially, the short box is based on the buying and selling of equivalent spread, and as long as the net premium received from the sale of the two spreads is substantially higher than the overall cost of establishing the spreads, an investor can lock in a risk-free profit.

In more detail, the short box is constructed by selling an in-the-money call option, buying and out-of-the-money call option, selling an in-the-money put option, and buying an out-of-the-money put option. This strategy is mainly used to take advantage of overpriced spreads. The overpricing occurs when the combined component prices exceed the combined vale of the spreads at expiration.