The base currency (also known as the transaction currency), is the first currency to appear in a currency pair quotation. For example, if you were looking at the EUR/USD currency pair, the euro would be the base currency and the U.S. dollar would be the quote currency.
The available base currencies are AUD, PLN, CAD, CHF, EUR, GBP, HKD, JPY, NZD, SGD, and USD.