Example

For example a GBP/USD FX option might be specified by a contract giving the owner the right but not the obligation to sell £1,000,000 and buy $2,000,000 on December 31. In this case the pre-agreed exchange rate, or strike price, is 2.0000 USD/GBP or 0.5000 GBP/USD and the notionals are £1,000,000 and $2,000,000 (£1,000,000 from the eyes of a USD investor, $2,000,000 from the eyes of a GBP investor).
This type of contract is both a call on dollars and a put on sterling, and is often called a USD/GBP put by market participants, as it is a put on the exchange rate; it could equally be called a GBP/USD call, but isn't, as market convention is to quote the 2.0000 number (normal quote), not the 0.5000 number (inverse quote).
If the rate is lower than 2.0000 USD/GBP come December 31 (say at 1.9000 USD/GBP), meaning that the dollar is stronger and the pound is weaker, then the option will be exercised, allowing the owner to sell GBP at 2.0000 and immediately buy it back in the spot market at 1.9000, making a profit of (2.0000 USD/GBP - 1.9000 USD/GBP)*1,000,000 GBP = 100,000 USD in the process. If they immediately exchange their profit into GBP this amounts to 100,000/1.9000 = 52,631.58 GBP.