A type of options strategy used when a decline in the price of the underlying asset is expected. It is achieved by selling call options at a specific strike price while also buying the same number of calls, but at a higher strike price. The maximum profit to be gained using this strategy is equal to the difference between the price paid for the long option and the amount collected on the short option.

For example, let's assume that a stock is trading at $30. An option investor has purchased one call option with a strike price of $35 for a premium of $0.50 and sold one call option with a strike price of $30 for a premium of $2.50. If the price of the underlying asset closes below $30 upon expiration, then the investor collects $200 (($2.50 - $0.50) * 100 shares/contract).