Contours relate when their imaginary extensions intersect at an obtuse or right angle.
However, it is shown here that contour relatability is neither necessary nor sufficient for completion to take place. A volume is defined as a 3-D enclosure. Typically, this refers to a surface plus the inside that it encloses. Two volumes are mergeable when their unbounded visible surfaces are relatable or the insides enclosed by those surfaces can completely merge. Two surfaces are relatable when their visible portions can be extended into occluded space along the trajectories defined by their respective curvatures so that they merge into a common surface.