Quad data rate (QDR, or quad pumping) is a communication signaling technique wherein data are transmitted at four points in the clock cycle: on the rising and falling edges, and at two intermediate points between them. The intermediate points are defined by a second clock that is 90° out of phase from the first. The effect is to deliver four bits of data per signal line per clock cycle. In a quad data rate system, the data lines operate at twice the frequency of the clock signal. This is in contrast to double data rate systems, in which the clock and data lines operate at the same frequency.