The classification and the distribution of building systems and the relation between climate, vegetation and distribution are reported in former reports. This is a report of the theory of the way of existence of building systems. We can recognise the species of building system as same as the biological world, when we survey the styles of building systems. The species of building systems exist as a community as a botanical community. So, some phenomenons of the way of existence of building systems are explained by the assistance of biological and ecological theory. The family tree of the species of building systems and even a kind of evolution in the historical change of the species are recognised. Then a new classification system of building systems unifying material, system, form is formed according to the theory, and 6 fundamental styles of building systems are abstructed. And at final stage, the 3 law of the distribution of building systems are explaned. 1. density- there are some high density communities called city. 2. similarity of nabours- nabour building systems have some similarity. 3, continuation and symmetry of distribution- styles of building systems change continuously and symmetrically.