  We derive commensurate scale relations which relate perturbatively calculable
QCD observables to each other, including the annihilation ratio, the heavy
quark potential, tau decay, and radiative corrections to structure function sum
rules. The ratio of scales $Q_A/Q_B$ is unique at leading order and guarantees
that the observables A and B pass through new quark thresholds at the same
physical scale. In higher orders a different renormalization scale $Q^{n*}$ is
assigned for each order n in the perturbative series such that the coefficients
of the series are identical to that of a conformally invariant theory. QCD can
thus be tested in a new and precise way by checking that the effective charges
of observables track both in their relative normalization and in their
commensurate scale dependence. The commensurate scale relations which relate
the radiative corrections to the annihilation ratio to the radiative
corrections for the Bjorken and Gross-Llewellyn Smith sum rules provides the
generalization of the Crewther relation to non-conformal gauge theory.
