  In a series of papers with Kossakowski, the first author has examined
properties of densities for which the positive partial transpositrionm (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 that same structure lends itself to checking the
necessary PPT condition and to a novel sufficient condition for separability.
