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For two disjoint sets A and B we have two possible situations: they can be complementary or mutually exclusive sets. If we denote by A'C the complement of A with regard to some larger set C containing A (i.e. A'C=C \ A, where A is a subset of C), it is also possible to take for C the universal set, so the complement of A is "no A". In this case we have the classical relation "figure"-"ground".


The interesting situation occurs in the case of neolithic black-white ornaments with a "figure" equivalent (congruent) to the "ground".

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