After a preliminary analysis of the concept of "gap (§ 1) and the assessment of particular gaps (cuts, breakings, § 2; mutilations, § 3), cases of extraneousness (§ 4) and of disintegration of the figure (§ 5) are presented. Multiple gaps (dashed contours, § 6) are considered, and their resemblance with illusion of partition is pointed out. Poggendorff’s illusion is brought back to a special case of misalignment due to a gap (§ 7). The filling up of gaps is also reviewed, in the forms of imagined operation (§ 8), amodal completion (§ 9) and of anomalous surfaces as well (§ 10). Some other effects of gaps on the aspect of the figure are illustrated (§ 11). General observations - even if provisional - are formulated, both on the study of optical-geometrical illusions and on their explanations (§ 12). The paper contains 55 historical figures in their original form and 30 new figures.