*Why do things look like they belong together? We know from psychological research that things tend to appear to belong together (1) when they appear to be similar (similarity grouping), (2) when they are near one another (proximity grouping), and (3) when one or more of their edges line up in space (continuity).
Any perceivable attribute is a potential basis for rhyme, including such charateristics as shape, size (thickness, length), light/dark value, hue (color), intensity (saturation), texture, direction, speed, reflectiveness, density and slant.