Reductive models $((\frak g,\frak h),H)$ for Cartan geometries are
showed to fall into two classes, symmetric and non symmetric type,
according to the existence or non existence of a mutation $\frak
g'=\frak h\oplus\frak m$ where the $H$-module $\frak m$ is an
abelian subalgebra. Sasakian structures are showed to be Cartan
geometries having a model of non symmetric type and other examples
of models of this type are exhibited. Reductive models for which
no Cartan space forms exist are constructed. The phenomenon of
non-existence of Cartan space forms pertains to models of non
symmetric type.