  We study the reducibility of parabolically induced representations of
non-split inner forms of quasi-split classical groups. The isomorphism of
Arthur R-groups, endoscopic R-groups and Knapp-Stein R-groups is established,
as well as showing these R-groups are isomorphic to the corresponding ones for
the quasi-split form. This shows R-groups are an invariant of the L-packets.
The results are applied to classify the elliptic spectrum.
