Briggs, F. 1990, Ap. J. 352, 15.
An analysis conducted for 12 galaxies with extended, warped H I disks in a variety of reference frames has led to the formulation of clear empirical criteria for galactic warp behavior. In view of these criteria, it emerges that while the H I layer is typically planar within R(25), warping becomes detectable within R(26.5); this is consistent with a straight line of the nodes (LON) measured in the plane defined by the innermost regions of the galaxies. At radii larger than R(26.5), the LON measured in the plane of the inner galaxy advances in the direction of galaxy rotation for successively larger radii. The nodes will accordingly lie along leading spirals in this frame of reference.