  Chen, Sederberg, and Zheng introduced the notion of a $\mu$-basis for a
rational ruled surface in Chen et al. (2001) and showed that its resultant is
the implicit equation of the surface, if the parametrization is generically
injective. We generalize this result to the case of an arbitrary
parametrization. We also give a new proof for the corresponding theorem in the
curve case and treat the reparametrization problem for curves and ruled
surfaces.
