  Commensurate scale relations are perturbative QCD predictions which relate
observable to observable at fixed relative scale, such as the ``generalized
Crewther relation", which connects the Bjorken and Gross-Llewellyn Smith deep
inelastic scattering sum rules to measurements of the $e^+ e^-$ annihilation
cross section. All non-conformal effects are absorbed by fixing the ratio of
the respective momentum transfer and energy scales. In the case of fixed-point
theories, commensurate scale relations relate both the ratio of couplings and
the ratio of scales as the fixed point is approached. The relations between the
observables are independent of the choice of intermediate renormalization
scheme or other theoretical conventions. Commensurate scale relations also
provide an extension of the standard minimal subtraction scheme, which is
analytic in the quark masses, has non-ambiguous scale-setting properties, and
inherits the physical properties of the effective charge $\alpha_V(Q^2)$
defined from the heavy quark potential. The application of the analytic scheme
to the calculation of quark-mass-dependent QCD corrections to the $Z$ width is
also reviewed.
