%e discuss the transformation from a scalar and vector Dirac equation to either a scalar and tensor or a vector and tensor equation in both the single-channel (static potential) and coupled-channel (nonstatic potential) cases. The relationship to the o.-co Lagrangian in the mean-field theory is pointed out. The failure to obtain such a transformation in the coupled-channel case, except when the states are degenerate, is explained simply and analyzed in some detail.