Widths of pull-aparts between strike-slip fault segments appear to obey power-law scaling relationships, at least up to the scale of hundreds of millimetres. Field data from the Liassic limestones of Somerset, UK, are presented and are discussed in relation to fluid flow through rock. The established scaling relationships are used to predict the contributions of different-sized pull-aparts to space created by strike-slip faults. Fluid-filled pull-aparts are likely to be significant in hydrocarbon reservoirs affected by strike-slip faulting because they are sub-vertical tube-like structures that may extend through reservoirs and top seals. There are, however, insufficient data to develop a model for permeability along pull-aparts. Models of fluid flow in rock typically assume a limited range of fracture apertures, so models can be improved by incorporating information on the scaling relationships of the fractures and the contributions of different-sized fractures to secondary porosity. Also, geomechanical models that assume planar strike-slip faults are likely to underestimate migration because they do not consider the reduction of compressive stresses at pull-aparts.