An evolution of DDR SDRAM, DDR2 allows four data transfers per clock cycle, by clocking the internal bus at twice the speed of the memory clock. Therefore the effective frequency of the memory becomes 4 times its actual frequency. A DDR2 800 module operates at 200 MHz. As in the case of DDR SDRAM, the alternate notation relying on the maximum data transfer speeds is also used. PC2 3200 refers to DDR2 400. A DDR2 module has 240 contacts, and is not backward-compatible.