[en] In a series of papers with Kossakowski, the first author has examined properties of densities for which the positive partial transpose (PPT) property can be readily checked. These densities were also investigated from a different perspective by Baumgartner, Hiesmayr and Narnhofer. In this paper, we show how the support of such densities can be expressed in terms of lines in a finite geometry and how the same structure lends itself to checking the necessary PPT condition and to a novel sufficient condition for separability