Sketch representation of the scattered light appearance of the PPD sample. Included are only the gaps and substructures considered to have been created by perturbers in Sect. 4. The adopted locations of these perturbers are shown as green dashed curves. Scattered-light rings are shown as thick rings, while arms and spiral patterns are shown as flat curled curves. The disks are projected according to their inclination and position angle values listed in Table 2. Also included are HD 100546, HD 139614 and CQ Tau for completeness, although no clear gaps or simulatedplanets creating the spiral pattern exist.