A fence or collar is an option strategy that establishes a trading band around a security or commodity, generally to protect profits. One form of a fence involves the sale of an out-of-the-money call option on an underlying security; all or part of the pmium thus received is used to buy a protective out-of-the money put on the security. Both the call and the put have the same expiration date. The call option establishes a ceiling price for the security, while the put option establishes a floor price for it, effectively 'fencing' in the option.
A widely used variant of this option strategy involves a "costless collar," where the pmium received through the sale of the call roughly equals the pmium paid for the purchase of the put. The cost of protection in this case is therefore zero.
For example, an investor who wishes to construct a fence or collar around a stock in the portfolio that is trading at $50 could sell a call with a strike price of $53, and buy a put with a strike price of $47, both with, say, three months to expiration. If the pmium received from the sale of the $53 call equals the pmium paid for the $47 put, this would be a "costless collar", and "fence" in potential losses and profits.