  Many researchers tried to understand/explain the geometric reasons for
paradoxical mobility of a mechanical linkage, i.e. the situation when a linkage
allows more motions than expected from counting parameters and constraints.
Bond theory is a method that aims at understanding paradoxical mobility from an
algebraic point of view. Here we give a self-contained introduction of this
theory and discuss its results on closed linkages with revolute or prismatic
joints.
