  The goal of this paper is to explain, using a typical example, the
distribution of the different assembly modes in the workspace and their
effective role in the execution of trajectories. The singular and non-singular
changes of assembly mode are described and compared to each other. The
non-singular change of assembly mode is more deeply analysed and discussed in
the context of trajectory planning. In particular, it is shown that, according
to the location of the initial and final configurations with respect to the
uniqueness domains in the workspace, there are three different cases to
consider before planning a linking trajectory.
