A flow regime characterized by parallel flow lines in the reservoir. This results from flow to a fracture or a long horizontal well, or from flow in an elongated reservoir, such as a fluvial channel, or as a formation bounded by parallel faults. Linear flow is recognized as a +1/2 slope in the pressure derivative on the log-log diagnostic plot. Its presence enables determination of the fracture half-length or the channel or reservoir width, if permeability can be determined independently.