This work demonstrates a lightness phenomenon useful to extend the notion of "belongingness," which is crucial to explain a class of illusions that include simultaneous lightness contrast, the Koffka-Benussi ring, the Benary cross, and the White effect. These phenomena manifest some kind of dissimilarity, difference, or change responsible for the perceived contrast. The dissimilarity is related to the "belongingness" of the crucial gray elements (i) to a unique or separated/divided object, as in the Koffka-Benussi ring, or (ii) to the figure or to the background, as in the Benary and White effects. If we plausibly assume that differences and changes are biologically important to be detected and if necessary highlighted, then any visible difference might induce a contrast effect. This is the main hypothesis demonstrated by the lightness phenomenon based on checks grouped vertically, split in two upper and lower halves, and segregated from the homogeneous gray background. The checks are alternated and vertically/horizontally reversed in the upper and lower halves of the pattern. Despite the constant visual organization and in spite of the identical local contrast within each check, the inner area of the elements of the upper group appears darker than the one of the lower group. The visible dissimilarity, although not related to the notion of belongingness, is sufficient to elicit a clear lightness difference.