  This paper characterizes geometrically the singularities of limited DOF
parallel manipulators. The geometric conditions associated with the dependency
of six Pl\"ucker vector of lines (finite and infinite) constituting the rows of
the inverse Jacobian matrix are formulated using Grassmann-Cayley algebra.
Manipulators under consideration do not need to have a passive spherical joint
somewhere in each leg. This study is illustrated with three example robots
