If we take the Euro and USD pair (EUR/USD) and assume (as an example) that today’s interest rates in Europe are far greater than those in America, and that we are holding a ‘Buy’ Position open overnight in the market, then with a Buy position, we Buy the base currency (EUR) and Sell the quote currency (USD). Therefore a deduction from the interest rate of the USD from that of the Euro, is applied. If the Euro has its interest rates at 3% and America has its rates at 0.5% you will be paid 2.5% (3% - 0.5%) interest, on roll over.