The notion that slip on faults is controlled by the balance between resolved shear stress and the magnitude of frictional resistance is examined in the light of the published orientations of ancient faults and slip lineations. By compiling data from more than 1500 faults used in 228 stress analyses, we observe that natural fault planes, as well as lineations formed by fault slip, display characteristic patterns of preferred orientation when rotated into the reference frame defined by the principal stress axes. These patterns vary systematically according to the relative magnitudes of principal stresses determined by fault-slip analysis. Furthermore, these patterns are strikingly similar to those predicted by a frictional model for fault reactivation in which the potential for slip is equated to the ratio of the components of shear stress and normal stress on the fault plane. Our findings therefore make a compelling case for the concept of frictional slip tendency for the prediction of fault reactivation and lend support to the validity of fault-slip methods for paleostress analysis.