  A closed 6R linkage is generically rigid. Special cases may be mobile. Many
families of mobile 6R linkages have been characterised in terms of the
invariant Denavit-Hartenberg parameters of the linkage. In other words, many
sufficient conditions for mobility are known. In this paper we give, for the
first time, equational conditions on the invariant Denavit-Hartenberg
parameters that are necessary for mobility. The method is based on the theory
of bonds. We illustrate the method by deriving the equational conditions for
various well-known linkages (Bricard's line symmetric linkage, Hooke's linkage,
Dietmaier's linkage, and recent a generalization of Bricard's orthogonal
linkage), starting from their bond diagrams; and by deriving the equations for
another bond diagram, thereby discovering a new mobile 6R linkage.
