Definitions
Currency rates are displayed by a pair of currencies participating in one transaction.
For instance, GBP/USD, USD/CHF, GPB/USD, where GPB/USD shows how many dollars is equal to 1 British Pound (how many dollars can be purchased for 1 British Pound) and USD/CHF shows how many Swiss Franks there are in one dollar (how many Swiss Franks can be purchased for 1 dollar).
The first currency is called basic currency (XXX or USD) and it determines how much of another national currency can be purchased for one unit of this basic currency.
Purchasing is always done by price Ask.