A relationship stating that the difference between the premiums of a call and a put with the same strike price and time to expiration equals the difference between the present value of the forward price and the present value of the strike price. C þ XerT ¼ P þ S; where C is defined as the call price per share; X is the strike price; P is the put price per share; S is the stock price per share; r is the risk-free rate and T is the constant period.