A major difficulty in introducing a discussion about the SDR or the European currency unit (ECU) is the fact that each can potentially refer to a number of different things. The SDR and the ECU are, first of all—and, in an important sense, primarily—units of account. As such, the manner in which the value of each unit is determined or established and its behavior over time are of primary importance. Both the SDR and the ECU have their values defined in reference to baskets of currencies. These baskets are generally reviewed and adjusted every five years.