The “double” in double data rate (DDR) comes from the manner in which data transfers on both the rising and falling edges of a memory module’s clock signal. However, whereas DDR memory is clocked at one-half the rate of the bus, DDR2 is clocked at one-quarter of that rate, effectively doubling the data transmission speed. Additionally, DDR2’s typical voltage dropped to 1.8V from original DDR’s 2.5V. Shown here is a DDR2-800 (PC2-6400) module.