This equality can be viewed as one of the faces of put-call symmetry. The reason is that the value of an option can be computed both in a domestic as well as in a foreign scenario. We consider the example of modelling the exchange rate of EUR/USD. In New York, the call option costs
USD and hence
EUR. This EUR-call option can also be viewed as a USD-put option with payoff
. This option costs
EUR in Frankfurt, because and have the same volatility. Of course, the New York value and the Frankfurt value must agree, which leads to (37).