Onset of warp in a disk in the case of a perturber outer to the disk (left panels) and inner to the disk (right panels); orange and green lines trace the orbit of the perturber. Systems at both 1 Myr and 20 Myr are shown as labelled in the panels. This onset is accompanied by spiral patterns that are not necessarily visible in the case where the disk is oriented (close to) edge-on (see upper panels) but are more readily visible in pole-on views (see bottom panels). In the case of an outer perturber, the spiral pattern and the onset of the warp evolve from the outer parts of the disk towards the inner parts, and conversely in the case of an inner perturber.