Let's assume that a stock is trading at $18 and an investor has purchased one call option with a strike price of $20 and sold one call option with a strike price of $25. If the price of the stock jumps up to $35, the investor must provide 100 shares to the buyer of the short call at $25. This is where the purchased call option allows the trader to buy the shares at $20 and sell them for $25, rather than buying the shares at the market price of $35 and selling them for a loss.
Investment dictionary. Academic. 2012.