The orientation of the principal axes of the g tensor with respect to the relationship of axial ligand planes to the porphyrin nitrogens has been studied in the framework of the,one-electron crystal field model for tetragonal and rhombic low-spin d(5) complexes such as ferriheme centers. All five d atomic orbitals were taken into account for two-different ground-state-electronic configurations, the "normal" (d(xy))(2)(d(xz),d(yz))(3) and the "novel" (d(xz),d(yz))(4)(d(xy))(1) configurations. The expressions for the g tensor, g values, and magnetic axes were derived on the basis of first-order perturbation theory. The conditions for co- and counterrotation of magnetic axes with rotation of planar axial ligands away from the porphyrin nitrogens toward the meso positions and beyond, as well as the order of g values, have been analyzed. It is found that counterrotation is the only possibility for the (d(xz),d(yz))(4)(d(xy))(1) configuration and that it is also by far more common for the (d(xy))(2)(d(xz),d(yz))(3) electron configuration. The possibilities of nonlinear co-/counterrotation are also explored. The predictions of this treatment are then compared to experimental results obtained from single-crystal EPR, glassy sample ESEEM, and solution NMR spectroscopic studies. It is clear that the majority of experimental systems reported thus far follow the major predictions of this treatment: Most systems exhibit angle-for-angle (linear) counterrotation of the g or chi tensor with rotation of planar axial ligands away for the N-Fe-N axes; Hence, knowing the structure of a model heme or heme protein, and in particular, the orientation of (fixed) axial ligand planes, one should be able to predict the approximate orientation of the in-plane magnetic axes. This knowledge provides a check on the values obtained in new solution NMR, single-crystal EPR or frozen solution ESEEM experiments.